scsi: ch: fixup refcounting imbalance for SCSI devices

The SCSI device is required to be present during ch_probe() and ch_open().
But the SCSI device itself is only checked during ch_open(), so it's anyones
guess if it had been present during ch_probe(). And consequently we can't
reliably detach it during ch_release(), as ch_remove() might have been
called first.  So initialize the changer device during ch_probe(), and take
a reference to the SCSI device during both ch_probe() and ch_open().

[mkp: fixed checkpatch warning]

Link: https://lore.kernel.org/r/20200213153207.123357-2-hare@suse.de


Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>