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