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