nvme-tcp: add nvme-tcp pdu size build protection

Make sure that we don't somehow mess up the wire structures in the spec.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kkch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>