fix several fallouts from r320156, ZFS ABD import
All of the problems were related to the FreeBSD-only features.
One was caused by a mismerge in the zfsbootcfg support code.
All others were in the TRIM support code.
Reported by: ken,
O. Hartmann <ohartmann@walstatt.org>, Trond Endrestøl <Trond.Endrestol@fagskolen.gjovik.no>
MFC after: 1 week
X-MFC with: r320156