mntfs: lock mntfs pseudo devfs vnode properly
Require devvp locked for mntfs_freevp(), to have it locked around
vgone(). Make that true for ffs, which is the only consumer of
the interface.
Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32761