cxgbe tom: Permit rcv_nxt mismatches on FIN for iSCSI connections on T6.

The remote peer might send a FIN in the middle of a burst of data
PDUs. In the case of T6 with data PDU completion moderation, the
driver would not have seen these PDUs since the final PDU in the burst
was never received resulting in a stale rcv_nxt when the FIN is
received.

While here, invert the logic in the condition to be more readable and
always set tp->rcv_nxt from the sequence number in the CPL. This sets
the proper value of rcv_nxt for FINs on connections with data received
but not reported via a CPL (e.g. a partial iSCSI PDU burst interrupted
by a FIN).

Reported by: Jithesh Arakkan @ Chelsio
Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D30871