Btrfs: don't add a NULL extended attribute

Passing a null extended attribute value means to remove the attribute,
but we don't have to add a new NULL extended attribute.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/xattr.c

@@ -122,6 +122,16 @@ static int do_setxattr(struct btrfs_trans_handle *trans,
 		 */
 		if (!value)
 			goto out;
+	} else {
+		di = btrfs_lookup_xattr(NULL, root, path, btrfs_ino(inode),
+					name, name_len, 0);
+		if (IS_ERR(di)) {
+			ret = PTR_ERR(di);
+			goto out;
+		}
+		if (!di && !value)
+			goto out;
+		btrfs_release_path(path);
 	}
 
 again: