Convert cryptostats to a counter_u64 array.

[markj]

Summary

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

These counters are of dubious usefulness to begin with, but I think
having a framework to easily add new counters is useful, so let's keep
them for now.

Note: we are changing the sysctl ABI, but it has changed once in HEAD
already since 12 was branched. (It was renamed in r359374.)

[cem]

I’m not keen on the struct <-> array coercion

sys/opencrypto/crypto.c

236

Sizeof mismatches the structure type

[markj]

Fix the internal counter array size.

[markj]

In D25466#562261, @cem wrote:

I’m not keen on the struct <-> array coercion

For what it's worth, this is how all of the network stats are handled as well.

Internally we use an array of counter_u64's and the sysctl handler for COUNTER_U64_ARRAY fetches the value for each stat out of the array. So really we are just assuming that struct cryptostats can be treated as an array of uint64_t.