malloc: retire mt_stats_zone in favor of pcpu_zone_64

[mjg]

Summary

The former was only created because the latter did not exist at the time.

[markj]

Did you consider adding a malloc_pcpu() instead of hard-coding the zone item size?

[mjg]

Yes, along with malloc_type equivalent. Using it here would be a little circular.

malloc_pcpu will possibly be written by me after malloc itself gets cleaned up. for example the zone selection method avoidably encodes size + pointer to a string in the same struct, making it less cache-friendly. once this is sorted out the same (cleaned up) method can be used for malloc_pcpu.

[markj]

So eventually mti_stats would be allocated using malloc_pcpu()? Ok. BTW, V_pfr_kentry_counter_z consumers would benefit from such a function as well. Right now it's a per-VNET zone but I think there's no good reason for that.