Change blist_next_leaf_alloc so that it can examine more than one leaf
after the one where the possible block allocation begins, and allocate
a larger number of blocks than the current limit. This does not affect
the limit on minimum allocation size, which still cannot exceed
BLIST_MAX_ALLOC.

Use this change to modify swp_pager_getswapspace and its callers, so
that they can allocate more than BLIST_MAX_ALLOC blocks if they are
available.

Tested by: pho
Approved by: markj (mentor)
Differential Revision: https://reviews.freebsd.org/D20579