Clear tp->tod in t4_pcb_detach().

Otherwise, a socket can have a non-NULL tp->tod while TF_TOE is clear.
In particular, if a newly accepted socket falls back to non-TOE due to
an active open failure, the non-TOE socket will still have tp->tod set
even though TF_TOE is clear.

Reviewed by:	np
MFC after:	2 weeks
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D27028

sys/dev/cxgbe/tom/t4_tom.c

@@ -382,6 +382,7 @@ t4_pcb_detach(struct toedev *tod __unused, struct tcpcb *tp)
 	}
 #endif
 
+	tp->tod = NULL;
 	tp->t_toe = NULL;
 	tp->t_flags &= ~TF_TOE;
 	toep->flags &= ~TPF_ATTACHED;