cxgbe/t4_tom: fixes for issues on the passive open side.
- Fix PR 227760 by getting the TOE to respond to the SYN after the call to toe_syncache_add, not during it. The kernel syncache code calls syncache_respond just before syncache_insert. If the ACK to the syncache_respond is processed in another thread it may run before the syncache_insert and won't find the entry. Note that this affects only t4_tom because it's the only driver trying to insert and expand syncache entries from different threads.
- Do not leak resources if an embryonic connection terminates at SYN_RCVD because of L2 lookup failures.
- Retire lctx->synq and associated code because there is never a need to walk the list of embryonic connections associated with a listener. The per-tid state is still called a synq entry in the driver even though the synq itself is now gone.
Sponsored by: Chelsio Communications