btrfs: send: simplify return logic from send_set_xattr()

There's no longer any need for the 'out' label as there are no resources
to cleanup anymore in case of an error and we can directly return if
begin_cmd() fails.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>