btrfs: xattr: fix attribute removal

An attribute is not removed by 'setfattr -x attr file' and remains
visible in attr list. This makes xfstests/062 pass again.

Signed-off-by: David Sterba <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/xattr.c

···
 		if (ret)
 			goto out;
 		btrfs_release_path(path);
+
+		/*
+		 * remove the attribute
+		 */
+		if (!value)
+			goto out;
 	}
 
 again:
···
 	return ret;
 }
 
+/*
+ * @value: "" makes the attribute to empty, NULL removes it
+ */
 int __btrfs_setxattr(struct btrfs_trans_handle *trans,
 		     struct inode *inode, const char *name,
 		     const void *value, size_t size, int flags)