uma: New check_align_mask(): Validate alignments (INVARIANTS)

uma: New check_align_mask(): Validate alignments (INVARIANTS)

New function check_align_mask() asserts (under INVARIANTS) that the mask
fits in a (signed) integer (see the comment) and that the corresponding
alignment is a power of two.

Use check_align_mask() in uma_set_align_mask() and also in uma_zcreate()
to replace the KASSERT() there (that was checking only for a power of
2).

Reviewed by:            kib, markj
MFC after:              2 weeks
Sponsored by:           The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D42263