scsi: sr: Fix sr_probe() missing mutex_destroy

If the device minor cannot be allocated or the cdrom fails to be registered
then the mutex should be destroyed.

Link: https://lore.kernel.org/r/06e9de38-eeed-1cab-5e08-e889288935b3@0882a8b5-c6c3-11e9-b005-00805fc181fe


Fixes: 51a85881 ("scsi: sr: get rid of sr global mutex")
Signed-off-by: Simon Arlott <simon@octiron.net>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>