mm: move get_page_from_free_area() to mm/page_alloc.c

The get_page_from_free_area() helper is only used in mm/page_alloc.c so
move it there to reduce noise in include/linux/mmzone.h

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Mike Rapoport (IBM) <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Acked-by: Kirill A. Shutemov <[e-mail]>
Reviewed-by: Matthew Wilcox (Oracle) <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/linux/mmzone.h

···
 	unsigned long		nr_free;
 };
 
-static inline struct page *get_page_from_free_area(struct free_area *area,
-					    int migratetype)
-{
-	return list_first_entry_or_null(&area->free_list[migratetype],
-					struct page, lru);
-}
-
 static inline bool free_area_empty(struct free_area *area, int migratetype)
 {
 	return list_empty(&area->free_list[migratetype]);

mm/page_alloc.c

···
 	zone->free_area[order].nr_free--;
 }
 
+static inline struct page *get_page_from_free_area(struct free_area *area,
+					    int migratetype)
+{
+	return list_first_entry_or_null(&area->free_list[migratetype],
+					struct page, lru);
+}
+
 /*
  * If this is not the largest possible page, check if the buddy
  * of the next-highest order is free. If it is, it's possible