uma: fix user build

[rlibby]

Summary

Rather than finding the right way to spell offsetof, just guard all of
slab definition in ifdef _KERNEL, since userspace doesn't need it.

Test Plan

make buildworld buildkernel

[rlibby]

I went ahead and committed this in r355746 along with re-application of D22759.

Marking abandoned because I don't see any other way to close it, having not mentioned the url in the log.