Reduce contention on per-adapter lock.

[jhb]

Summary

• 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).

Test Plan

• tested with KTLS RX where it made a marginal perf improvement (29 Gbps to 30-31 Gbps). However, in pmcstat callchains, lock_delay due to lock contention went from 6% of overall time (of which 82% was for the adapter lock in ccr_process) to 3% of overall time (of which 3% was for the per-session lock in ccr_process)