powerpc/vdso: Add missing includes and clean vdso_setup_syscall_map()

Instead of including extern references locally in
vdso_setup_syscall_map(), add the missing headers.

sys_ni_syscall() being a function, cast its address to
an unsigned long instead of declaring it as a fake
unsigned long object.

At the same time, remove a comment which paraphrases the
function name.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b4afedce748ed2858299ceab5ae29b52109263ef.1601197618.git.christophe.leroy@csgroup.eu

arch/powerpc/kernel/vdso.c

@@ -17,8 +17,10 @@
 #include <linux/elf.h>
 #include <linux/security.h>
 #include <linux/memblock.h>
+#include <linux/syscalls.h>
 #include <vdso/datapage.h>
 
+#include <asm/syscall.h>
 #include <asm/processor.h>
 #include <asm/mmu.h>
 #include <asm/mmu_context.h>
@@ -639,24 +641,18 @@ static __init int vdso_setup(void)
 static void __init vdso_setup_syscall_map(void)
 {
 	unsigned int i;
-	extern unsigned long *sys_call_table;
-#ifdef CONFIG_PPC64
-	extern unsigned long *compat_sys_call_table;
-#endif
-	extern unsigned long sys_ni_syscall;
-
 
 	for (i = 0; i < NR_syscalls; i++) {
 #ifdef CONFIG_PPC64
-		if (sys_call_table[i] != sys_ni_syscall)
+		if (sys_call_table[i] != (unsigned long)&sys_ni_syscall)
 			vdso_data->syscall_map_64[i >> 5] |=
 				0x80000000UL >> (i & 0x1f);
 		if (IS_ENABLED(CONFIG_COMPAT) &&
-		    compat_sys_call_table[i] != sys_ni_syscall)
+		    compat_sys_call_table[i] != (unsigned long)&sys_ni_syscall)
 			vdso_data->syscall_map_32[i >> 5] |=
 				0x80000000UL >> (i & 0x1f);
 #else /* CONFIG_PPC64 */
-		if (sys_call_table[i] != sys_ni_syscall)
+		if (sys_call_table[i] != (unsigned long)&sys_ni_syscall)
 			vdso_data->syscall_map_32[i >> 5] |=
 				0x80000000UL >> (i & 0x1f);
 #endif /* CONFIG_PPC64 */
@@ -738,9 +734,6 @@ static int __init vdso_init(void)
 #endif
 
 
-	/*
-	 * Setup the syscall map in the vDOS
-	 */
 	vdso_setup_syscall_map();
 
 	/*