geom_vfs: lock devvp in g_vfs_close()
It is needed for g_vfs_close() invalidating the buffers. We rely on the
vnode lock for correctness.
Reported and tested by: pho
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D32761