memstat_kvm_uma: fix reading of uma_zone_domain structures

[vangyzen]

Summary

Coverity flagged the scaling by sizeof(uzd). That is the type
of the pointer, so the scaling was already done by pointer arithmetic.
However, this was also passing a stack frame pointer to kvm_read,
so it was doubly wrong.

Move ZDOM_GET into the !_KERNEL section and use it in libmemstat.

Reported by: Coverity

Test Plan

I ran this against a vmcore file and it still printed some garbage
stats, but the printf I added for debugging no longer complained.

[markj]

sys/vm/uma_int.h

531 ↗

(On Diff #76284)

Fix the indent to be consistent while you're here?