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>

mm/swap_state.c

@@ -28,7 +28,9 @@
 static const struct address_space_operations swap_aops = {
 	.writepage	= swap_writepage,
 	.set_page_dirty	= swap_set_page_dirty,
+#ifdef CONFIG_MIGRATION
 	.migratepage	= migrate_page,
+#endif
 };
 
 static struct backing_dev_info swap_backing_dev_info = {