f2fs: cleanup waiting routine for writeback pages in cp

use genernal method supported by kernel

 o changes from v1
   If any waiter exists at end io, wake up it.

Signed-off-by: Changman Lee <cm224.lee@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 625eb4b..89dc750 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -372,7 +372,7 @@
 	struct mutex writepages;		/* mutex for writepages() */
 	bool por_doing;				/* recovery is doing or not */
 	bool on_build_free_nids;		/* build_free_nids is doing */
-	struct task_struct *cp_task;		/* checkpoint task */
+	wait_queue_head_t cp_wait;
 
 	/* for orphan inode management */
 	struct list_head orphan_inode_list;	/* orphan inode list */