Allow to create swap zone larger than v_page_count / 2.

If user configured the maxswapzone tunable, just take the literal
value for the initial zone sizing attempt. Before, it was only
possible to reduce the zone by the tunable.

While there, correct the message which was not correct when zone
creation rounded the size up.

Reported by: jmg
Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D18381