f2fs: use ihold

Use the following helper function committed by Al.

commit 7de9c6ee
Author: Al Viro <viro@zeniv.linux.org.uk>
Date:   Sat Oct 23 11:11:40 2010 -0400

    new helper: ihold()

...
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/namei.c

@@ -172,7 +172,7 @@ static int f2fs_link(struct dentry *old_dentry, struct inode *dir,
 	f2fs_balance_fs(sbi);
 
 	inode->i_ctime = CURRENT_TIME;
-	atomic_inc(&inode->i_count);
+	ihold(inode);
 
 	set_inode_flag(F2FS_I(inode), FI_INC_LINK);
 	ilock = mutex_lock_op(sbi);