Increase the pageout cluster size to 32 pages.

Decouple the pageout cluster size from the size of the hash table entry
used by the swap pager for mapping (object, pindex) to a block on the
swap device(s), and keep the size of a hash table entry at its current
size.

Eliminate a pointless macro.

Reviewed by: kib, markj (an earlier version)
MFC after: 4 weeks
Differential Revision: https://reviews.freebsd.org/D11305