UMA: unsign some variables related to allocation in hash_alloc().

As a followup to r343673, unsign some variables related to allocation
since the hashsize cannot be negative. This gives a bit more space to
handle bigger allocations and avoid some implicit casting.

While here also unsign uh_hashmask, it makes little sense to keep that
signed.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D19148