uma: Hide 'uma_align_cache'; Create/rename accessors

Create the uma_get_cache_align_mask() accessor and put it in a separate
private header so as to minimize namespace pollution in header/source
files that need only this function and not the whole 'uma.h' header.

Make sure the accessors have '_mask' as a suffix, so that callers are
aware that the real alignment is the power of two that is the mask plus
one. Rename the stem to something more explicit. Rename
uma_set_cache_align_mask()'s single parameter to 'mask'.

Hide 'uma_align_cache' to ensure that it cannot be set in any other way
then by a call to uma_set_cache_align_mask(), which will perform sanity
checks in a further commit. While here, rename it to
'uma_cache_align_mask'.

This is also in preparation for some further changes, such as improving
the sanity checks, eliminating internal resolving of UMA_ALIGN_CACHE and
changing the type of the 'uma_cache_align_mask' variable.

Reviewed by: markj, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42258