Remove remnants of optimization for > pagesize allocations.

In the past, this allocator seems to have allocated things larger than
a page seperately. Much of this code was removed at some point (perhaps
along with sbrk() used) so remove the rest. Instead, keep allocating in
power-of-two bins up to FIRST_BUCKET_SIZE << (NBUCKETS - 1). If we want
something more efficent, we should use a fancier allocator.

While here, remove some vestages of sbrk() use. Most importantly, don't
try to page align the pagepool since it's always page aligned by mmap().

Reviewed by: kib
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21453