Improve handling of rdev->si_mountpt on mount and unmount of FFS
volumes. Treat the field as a semaphore protecting availability of
the device for mounting. Do no access devvp->v_rdev without the vnode
lock owned.

Protect change of the devvp->v_bufobj bo_ops vector with the vnode
lock.

Reviewed by: bde
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks