Make domainset iterators unsigned.

[markj]

Summary

Otherwise iter % ds->ds_cnt will be negative once iter wraps around to a
negative number.

Test Plan

Peter found the problem while testing some other NUMA patches, one
of which adds a domainset iterator to each UMA keg.

It seems that the per-thread and per-object iterators never managed
to reach 0x80000000 in previous testing.