A new parameter to blist_alloc specifies an upper bound on the size of
the allocation request, so that the blocks allocated are from the next
set of free blocks big enough to satisfy the minimum requirements of
the request, and the number of blocks allocated are as many as
possible, up to the specified maximum. The implementation of
swp_pager_getswapspace uses this parameter to ask for a number of
blocks between the new halved request size and the previous failed
request size. Thus a request for 32 blocks may fail, but instead of
getting only 16 blocks instead, the caller asks for 16 to 31 next, and
might get 19 or 27, which is closer to what they originally wanted.

I expect this to lead to bigger block allocations and less block
fragmentation, at least in some cases.

Approved by: kib (mentor)
Differential Revision: https://reviews.freebsd.org/D20001