Fix rare double free in vdev_geom_attrchanged

Fix rare double free in vdev_geom_attrchanged

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/vdev_geom.c
	Don't drop the g_topology_lock before freeing old_physpath. That
	opens up a race where one thread can call vdev_geom_attrchanged,
	set old_physpath, drop the g_topology_lock, then block trying to
	acquire the SCL_STATE lock. Then another thread can come into
	vdev_geom_attrchanged, set old_physpath to the same value, and
	proceed to free it. When the first thread resumes, it will free
	the same location.

	It turns out that the SCL_STATE lock isn't needed. It was
	originally added by gibbs to protect vd->vdev_physpath while
	updating the same. However, the update process subsequently was
	switched to an atomic operation (a pointer swap). Now, there is
	no need for the SCL_STATE lock, and hence no need to drop the
	g_topology_lock.

Reviewed by:	delphij
MFC after:	4 weeks
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D5413