cxgbe tom: Support sending "raw" WR mbufs on plain TCP and TLS sockets

This is intended to support control work requests that manipulate
connection state but do not transmit data. Raw WRs are transmitted
immediately if possible, otherwise they are queued to an mbufq in the
toe pcb until sufficient credits are available. Raw WRs take
precendence over transmitting socket data.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D47761

(cherry picked from commit e2a2a7581c9663a08b8d92a34040d6fcdfc20816)