zvol_os: Fix handling of zvol private data

zvol private data is supposed to be nulled by zvol_clear_private before
zvol_free is called as an indicator that the zvol is going away.

Implement zvol_clear_private for volmode=dev.

Assert that zvol_clear_private has been called before zvol_free.

Check that zvol_clear_private has not been called when updating
volsize. If it has, fail with ENXIO.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11117