cxgbe/t4_tom: Detach the toep from the tcpcb when entering TIME_WAIT.

[np]

Summary

The kernel used to call tod_pcb_detach when entering TIME_WAIT but that
seems to have changed, likely with the TIME_WAIT overhaul in the kernel
some time ago. Catch up by having the driver perform the detach.

The hardware does not handle TIME_WAIT so it's important to detach and
let the kernel arm the 2MSL timer to deal with it.

Reported by: Sony Arpita Das @ Chelsio

[jhb]

In commit 0d7445193abc7c68703fd9b6de39f3f6cf6b55c9, tcp_twstart() stopped calling tcp_discardcb(), and tcp_discardcb() is what was calling tod_pcb_detach. I guess in theory when time wait completes and the inp is freed is when detach would get called? Another possible fix would be to change tcp_twstart() to explicitly call tcp_offload_detach()? That would fix T3 and not just T4. Alternatively, we could add a new tod_twstart hook and in then in the T4 TOM driver we could have that call t4_pcb_detach?