cxgbe/t4_tom: fixes for issues on the passive open side.

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.

PR:		227760
MFC after:	2 weeks
Sponsored by:	Chelsio Communications