uma: Make the cache alignment mask unsigned

uma: Make the cache alignment mask unsigned

In uma_set_align_mask(), ensure that the passed value doesn't have its
highest bit set, which would lead to problems since keg/zone alignment
is internally stored as signed integers.  Such big values do not make
sense anyway and indicate some programming error.  A future commit will
introduce checks for this case and other ones.

Reviewed by:            kib, markj
Sponsored by:           The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D42262

(cherry picked from commit 3d8f548b9e5772ff6890bdc01f7ba7b76203857d)

Approved by:    markj (mentor)