cxgbe/t4_tom: Add synq entry to the list before calling send_synack.

This fixes a panic where the peer's ack to the synack arrives on a
different queue and do_pass_establish tries to remove the synqe from
synqe_list before it has been added by do_pass_accept_req.

Reported by: Sony Arpita Das @ Chelsio
Fixes: 283333c0e329 cxgbe/t4_tom: Track all synq entries in a per-adapter list.
MFC after: 1 week
Sponsored by: Chelsio Communications