Reduce contention on per-adapter lock.

Reduce contention on per-adapter lock.

- Move temporary sglists into the session structure and protect them
  with a per-session lock instead of a per-adapter lock.

- Retire an unused session field, and move a debugging field under
  INVARIANTS to avoid using the session lock for completion handling
  when INVARIANTS isn't enabled.

- Use counter_u64 for per-adapter statistics.

Note that this helps for cases where multiple sessions are used
(e.g. multiple IPsec SAs or multiple KTLS connections).  It does not
help for workloads that use a single session (e.g. a single GELI
volume).

Reviewed by:	np
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D25457