Convert cryptostats to a counter_u64 array.

The global counters were not SMP-friendly. Use per-CPU counters
instead.

Reviewed by: jhb
Sponsored by: Rubicon Communications, LLC (Netgate)
Differential Revision: https://reviews.freebsd.org/D25466

(cherry picked from commit 7290cb47fced28b5d0654cd1de7cc721e214ea32)