powerpc: Force inlining of mmu_has_feature to fix build failure

The test robot has managed to generate a random config leading
to following build failure:

  LD      .tmp_vmlinux.kallsyms1
powerpc64-linux-ld: arch/powerpc/mm/pgtable.o: in function `ptep_set_access_flags':
pgtable.c:(.text.ptep_set_access_flags+0xf0): undefined reference to `hash__flush_tlb_page'
powerpc64-linux-ld: arch/powerpc/mm/book3s32/mmu.o: in function `MMU_init_hw_patch':
mmu.c:(.init.text+0x452): undefined reference to `patch__hash_page_A0'
powerpc64-linux-ld: mmu.c:(.init.text+0x45e): undefined reference to `patch__hash_page_A0'
powerpc64-linux-ld: mmu.c:(.init.text+0x46a): undefined reference to `patch__hash_page_A1'
powerpc64-linux-ld: mmu.c:(.init.text+0x476): undefined reference to `patch__hash_page_A1'
powerpc64-linux-ld: mmu.c:(.init.text+0x482): undefined reference to `patch__hash_page_A2'
powerpc64-linux-ld: mmu.c:(.init.text+0x48e): undefined reference to `patch__hash_page_A2'
powerpc64-linux-ld: mmu.c:(.init.text+0x49e): undefined reference to `patch__hash_page_B'
powerpc64-linux-ld: mmu.c:(.init.text+0x4aa): undefined reference to `patch__hash_page_B'
powerpc64-linux-ld: mmu.c:(.init.text+0x4b6): undefined reference to `patch__hash_page_C'
powerpc64-linux-ld: mmu.c:(.init.text+0x4c2): undefined reference to `patch__hash_page_C'
powerpc64-linux-ld: mmu.c:(.init.text+0x4ce): undefined reference to `patch__flush_hash_A0'
powerpc64-linux-ld: mmu.c:(.init.text+0x4da): undefined reference to `patch__flush_hash_A0'
powerpc64-linux-ld: mmu.c:(.init.text+0x4e6): undefined reference to `patch__flush_hash_A1'
powerpc64-linux-ld: mmu.c:(.init.text+0x4f2): undefined reference to `patch__flush_hash_A1'
powerpc64-linux-ld: mmu.c:(.init.text+0x4fe): undefined reference to `patch__flush_hash_A2'
powerpc64-linux-ld: mmu.c:(.init.text+0x50a): undefined reference to `patch__flush_hash_A2'
powerpc64-linux-ld: mmu.c:(.init.text+0x522): undefined reference to `patch__flush_hash_B'
powerpc64-linux-ld: mmu.c:(.init.text+0x532): undefined reference to `patch__flush_hash_B'
powerpc64-linux-ld: arch/powerpc/mm/book3s32/mmu.o: in function `update_mmu_cache':
mmu.c:(.text.update_mmu_cache+0xa0): undefined reference to `add_hash_page'
powerpc64-linux-ld: mm/memory.o: in function `zap_pte_range':
memory.c:(.text.zap_pte_range+0x160): undefined reference to `flush_hash_pages'
powerpc64-linux-ld: mm/memory.o: in function `handle_pte_fault':
memory.c:(.text.handle_pte_fault+0x180): undefined reference to `hash__flush_tlb_page'

This is due to mmu_has_feature() not being inlined. See extract of build of
mmu.c with -Winline:

In file included from ./include/linux/mm_types.h:19,
                 from ./include/linux/mmzone.h:21,
                 from ./include/linux/gfp.h:6,
                 from ./include/linux/mm.h:10,
                 from arch/powerpc/mm/book3s32/mmu.c:21:
./arch/powerpc/include/asm/mmu.h: In function 'find_free_bat':
./arch/powerpc/include/asm/mmu.h:231:20: warning: inlining failed in call to 'early_mmu_has_feature': call is unlikely and code size would grow [-Winline]
  231 | static inline bool early_mmu_has_feature(unsigned long feature)
      |                    ^~~~~~~~~~~~~~~~~~~~~
./arch/powerpc/include/asm/mmu.h:291:9: note: called from here
  291 |  return early_mmu_has_feature(feature);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The code relies on constant folding of MMU_FTRS_POSSIBLE at buildtime
and elimination of non possible parts of code at compile time.
For this to work, mmu_has_feature() and early_mmu_has_feature()
must be inlined.

Fixes: 259149c ("powerpc/32s: Only build hash code when CONFIG_PPC_BOOK3S_604 is selected")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/cf61345912c078c96f171afd0fcc48ef27cbdc3f.1614443418.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/mmu.h

@@ -228,7 +228,7 @@ enum {
 #define MMU_FTRS_ALWAYS		0
 #endif
 
-static inline bool early_mmu_has_feature(unsigned long feature)
+static __always_inline bool early_mmu_has_feature(unsigned long feature)
 {
 	if (MMU_FTRS_ALWAYS & feature)
 		return true;
@@ -286,7 +286,7 @@ static inline void mmu_feature_keys_init(void)
 
 }
 
-static inline bool mmu_has_feature(unsigned long feature)
+static __always_inline bool mmu_has_feature(unsigned long feature)
 {
 	return early_mmu_has_feature(feature);
 }