mm/page_alloc: make should_fail_alloc_page() static

make W=1 generates the following warning for mm/page_alloc.c

  mm/page_alloc.c:3651:15: warning: no previous prototype for `should_fail_alloc_page' [-Wmissing-prototypes]
   noinline bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
                 ^~~~~~~~~~~~~~~~~~~~~~

This function is deliberately split out for BPF to allow errors to be
injected.  The function is not used anywhere else so it is local to the
file.  Make it static which should still allow error injection to be used
similar to how block/blk-core.c:should_fail_bio() works.

Link: https://lkml.kernel.org/r/20210520084809.8576-4-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3819,7 +3819,7 @@ static inline bool __should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
 
 #endif /* CONFIG_FAIL_PAGE_ALLOC */
 
-noinline bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
+static noinline bool should_fail_alloc_page(gfp_t gfp_mask, unsigned int order)
 {
 	return __should_fail_alloc_page(gfp_mask, order);
 }