shutdown: unmount filesystems after swapoff
Swap on file requires operational underlying mount, otherwise
swapoff_all() is guaranteed to panic due to the default strategy VOP for
reclaimed vnodes.
Reported and tested by: peterj
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33147