mm/page_alloc: move prototype for find_suitable_fallback

make W=1 generates the following warning in mmap_lock.c for allnoconfig

  mm/page_alloc.c:2670:5: warning: no previous prototype for `find_suitable_fallback' [-Wmissing-prototypes]
   int find_suitable_fallback(struct free_area *area, unsigned int order,
       ^~~~~~~~~~~~~~~~~~~~~~

find_suitable_fallback is only shared outside of page_alloc.c for
CONFIG_COMPACTION but to suppress the warning, move the protype outside of
CONFIG_COMPACTION.  It is not worth the effort at this time to find a
clever way of allowing compaction.c to share the code or avoid the use
entirely as the function is called on relatively slow paths.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Mel Gorman <[email protected]>
Reviewed-by: Yang Shi <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Cc: Dan Streetman <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/internal.h

@@ -274,11 +274,10 @@ isolate_freepages_range(struct compact_control *cc,
 int
 isolate_migratepages_range(struct compact_control *cc,
 			   unsigned long low_pfn, unsigned long end_pfn);
+#endif
 int find_suitable_fallback(struct free_area *area, unsigned int order,
 			int migratetype, bool only_stealable, bool *can_steal);
 
-#endif
-
 /*
  * This function returns the order of a free page in the buddy system. In
  * general, page_zone(page)->lock must be held by the caller to prevent the