nvmf_tcp: Ensure the PDU length is long enough for the header digest

[jhb]

Summary

Header digests are optional, but if they are present, the PDU length
(plen) must be at least long enough to include both the full header
(ch->hlen) and the header digest. Previously, a header-only PDU with
the HDGST flag set but omitting the actual digest from the PDU length
could cause the kernel to attempt to read the received digest off the
end of an mbuf resulting in a panic.