ffs: fix newly introduced LOR between mntfs vnode lock and topology lock

The mntfs vnode lock should be before topology, as established in
ffs_mountfs(). Extend the locked region in ffs_unmount().

Reported and reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33013