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