Fix uma boot pages calculations on NUMA machines that also don't have
MD_UMA_SMALL_ALLOC. This is unusual but not impossible. Fix the alignemnt
of zones while here. This was already correct because uz_cpu strongly
aligned the zone structure but the specified alignment did not match
reality and involved redundant defines.

Reviewed by: markj, rlibby
Differential Revision: https://reviews.freebsd.org/D23046