btrfs: fix missing error return in btrfs_drop_snapshot

If btrfs_del_root fails in btrfs_drop_snapshot, we'll pick up the
error but then return 0 anyway due to mixing err and ret.

Fixes: 79787ea ("btrfs: replace many BUG_ONs with proper error handling")
Cc: <stable@vger.kernel.org> # v3.4+
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -9220,6 +9220,7 @@ int btrfs_drop_snapshot(struct btrfs_root *root,
 	ret = btrfs_del_root(trans, fs_info, &root->root_key);
 	if (ret) {
 		btrfs_abort_transaction(trans, ret);
+		err = ret;
 		goto out_end_trans;
 	}