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