scsi: ch: Make it possible to open a ch device multiple times again

Clearing ch->device in ch_release() is wrong because that pointer must
remain valid until ch_remove() is called. This patch fixes the following
crash the second time a ch device is opened:

BUG: kernel NULL pointer dereference, address: 0000000000000790
RIP: 0010:scsi_device_get+0x5/0x60
Call Trace:
 ch_open+0x4c/0xa0 [ch]
 chrdev_open+0xa2/0x1c0
 do_dentry_open+0x13a/0x380
 path_openat+0x591/0x1470
 do_filp_open+0x91/0x100
 do_sys_open+0x184/0x220
 do_syscall_64+0x5f/0x1a0
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: 085e5676 ("scsi: ch: add refcounting")
Cc: Hannes Reinecke <hare@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191009173536.247889-1-bvanassche@acm.orgReported-by: Rob Turk <robtu@rtist.nl>
Suggested-by: Rob Turk <robtu@rtist.nl>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/ch.c

@@ -579,7 +579,6 @@ ch_release(struct inode *inode, struct file *file)
 	scsi_changer *ch = file->private_data;
 
 	scsi_device_put(ch->device);
-	ch->device = NULL;
 	file->private_data = NULL;
 	kref_put(&ch->ref, ch_destroy);
 	return 0;