linuxkpi: dma_get_cache_alignment(): Fix off-by-one result

Substituting 'uma_align_cache' by the appropriately named accessor
uma_get_cache_align_mask() made apparent that dma_get_cache_alignment()
was off by one, since it was defined to be the mask derived from the
alignment value.

Reviewed by: markj, bz
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42264