Passing UMA_ZONE_NOFREE to uma_zcreate() for swpctrie_zone and swblk_zone is
redundant, because uma_zone_reserve_kva() is performed on both zones and it
sets this same flag on the zone. (Moreover, the implementation of the swap
pager does not itself require these zones to be UMA_ZONE_NOFREE.)

Reviewed by: kib, markj
Approved by: re (gjb)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D17296