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>

fs/f2fs/super.c

@@ -876,6 +876,7 @@ static int f2fs_fill_super(struct super_block *sb, void *data, int silent)
 	spin_lock_init(&sbi->stat_lock);
 	init_rwsem(&sbi->bio_sem);
 	init_rwsem(&sbi->cp_rwsem);
+	init_waitqueue_head(&sbi->cp_wait);
 	init_sb_info(sbi);
 
 	/* get an inode for meta space */