include/linux/migrate.h: remove migrate_page #define

This is designed to avoid a few ifdefs in .c files but it's obnoxious
because it can cause unsuspecting "migrate_page" symbols to get turned into
"NULL".

Just nuke it and use the ifdefs.

Cc: Konstantin Khlebnikov <k.khlebnikov@samsung.com>
Cc: Rafael Aquini <aquini@redhat.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/shmem.c b/mm/shmem.c
index 469f90d..4fad61b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -3077,7 +3077,9 @@
 	.write_begin	= shmem_write_begin,
 	.write_end	= shmem_write_end,
 #endif
+#ifdef CONFIG_MIGRATION
 	.migratepage	= migrate_page,
+#endif
 	.error_remove_page = generic_error_remove_page,
 };