vfs: Unmount /proc to release refcnts pertained to root

/proc must be unmounted to release refcnts which pertains to the root
mountpoint, i.e. zfs.
It was preventing zfs_umount from releasing the mp dentries properly,
thus VOP_INACTIVE from being called on the respective vnodes.

Found the problem while dumping the mountpoint refcnts.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>