mm: apply __must_check to vmap_pages_range_noflush()

To prevent errors when vmap_pages_range_noflush() or
__vmap_pages_range_noflush() silently fail (see the link below for an
example), annotate them with __must_check so that the callers do not
unconditionally assume the mapping succeeded.

Reported-by: Dipanjan Das <mail.dipanjan.das@gmail.com>
Link: https://lore.kernel.org/linux-mm/CANX2M5ZRrRA64k0hOif02TjmY9kbbO2aCBPyq79es34RXZ=cAw@mail.gmail.com/
Signed-off-by: Alexander Potapenko <glider@google.com>

mm/internal.h

@@ -833,20 +833,20 @@ size_t splice_folio_into_pipe(struct pipe_inode_info *pipe,
  * mm/vmalloc.c
  */
 #ifdef CONFIG_MMU
-int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
-                pgprot_t prot, struct page **pages, unsigned int page_shift);
+int __must_check vmap_pages_range_noflush(unsigned long addr, unsigned long end,
+		pgprot_t prot, struct page **pages, unsigned int page_shift);
 #else
 static inline
-int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
-                pgprot_t prot, struct page **pages, unsigned int page_shift)
+int __must_check vmap_pages_range_noflush(unsigned long addr, unsigned long end,
+		pgprot_t prot, struct page **pages, unsigned int page_shift)
 {
 	return -EINVAL;
 }
 #endif
 
-int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
-			       pgprot_t prot, struct page **pages,
-			       unsigned int page_shift);
+int __must_check __vmap_pages_range_noflush(
+	unsigned long addr, unsigned long end, pgprot_t prot,
+	struct page **pages, unsigned int page_shift);
 
 void vunmap_range_noflush(unsigned long start, unsigned long end);