Restrict supported alignment for malloc_domainset_aligned(9) to PAGE_SIZE.

UMA page_alloc() does not take an alignment, so UMA can only handle
alignment less then page size.

Noted by: alc
Reviewed by: alc, markj (previous version)
Discussed with: jrtc27
Tested by: pho (previous version)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28219