Micro-optimize uma_small_alloc(). Replace bzero(..., PAGE_SIZE) by
pagezero(). Ultimately, they use the same method for bulk zeroing, but
the generality of bzero() requires size and alignment checks that
pagezero() does not.

Eliminate an unnecessary #include.

Reviewed by: emaste, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26876