linux/zvol_os: don't try to set disk ops if alloc fails

If the kernel fails to allocate the gendisk, zvo_disk will be NULL, and
derefencing it will explode. So don't do that.

Sponsored-by: Klara, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17396