The old code had a O(n) loop, where n is the size of /dev/devstat.

Multiply that by another O(n) loop in devstat_mmap for a total of

O(n^2).

This change adds DIOCGMEDIASIZE support to /dev/devstat so userland can

quickly determine the right amount of memory to map, eliminating the

O(n) loop in userland.

This change decreases the time to run "gstat -bI0.001" with 16,384 md

devices from 29.7s to 4.2s.

MFC after: 2 weeks

Sponsored by: Axcient

[skip ci] fix a typo in a comment in mdconfig.c Sponsored by: Axcient