gctl_get_geom: Skip validation of g_class.

[delphij]

Summary

The caller from kernel is expected to provide an valid g_class
pointer, instead of traversing the global g_class list, just
assert that it is not NULL and use it directly instead.

MFC after: 2 weeks

[mav]

Makes sense. Not sure why it was made so paranoid.

[phk]

It was primarily made paranoid because GEOM went into the kernel during a very turbulent period in terms of locking, and because some of the compatibility shims necessary to keep systems working were ... "interesting".