MFC r227110,r227111:

r227110:

In zvol_open() if the spa_namespace_lock is already held, it means that
ZFS is trying to open and taste ZVOL as its VDEV. This is not supported,
so return an error instead of panicing on spa_namespace_lock recursion.

Reported by: Robert Millan <rmh@debian.org>
PR: kern/162008

r227111:

Correct typo in comment.

Reported by: Fabian Keil <fk@fabiankeil.de>

Approved by: re (kib)