scsi: core: check for device state in __scsi_remove_target()

As it turned out device_get() doesn't use kref_get_unless_zero(), so we
will be always getting a device pointer.  Consequently, we need to check
for the device state in __scsi_remove_target() to avoid tripping over
deleted objects.

Fixes: fbce4d97 ("scsi: fixup kernel warning during rmmod()")
Reported-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>