page migration: fix wrong comment in address_space_operations.migratepage()

There is no parameter "sync" in address_space_operations->migratepage().
It should be migrate_mode.  And the comment is for MIGRATE_ASYNC.
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/fs.h

@@ -372,8 +372,8 @@ struct address_space_operations {
 	int (*get_xip_mem)(struct address_space *, pgoff_t, int,
 						void **, unsigned long *);
 	/*
-	 * migrate the contents of a page to the specified target. If sync
-	 * is false, it must not block.
+	 * migrate the contents of a page to the specified target. If
+	 * migrate_mode is MIGRATE_ASYNC, it must not block.
 	 */
 	int (*migratepage) (struct address_space *,
 			struct page *, struct page *, enum migrate_mode);