You should also remove the cmd's 'nr_mapped' field while you're at it,
otherwise you'll hit the WARN in nvmet_tcp_free_cmd_buffers().
Not remove nr_mapped because we use it to know the iovec entries, but
we can just remove the WARN statement.
It's only used locally within nvmet_tcp_map_pdu_iovec() after this change, so
no need to carry it in struct nvmet_tcp_cmd anymore.