scsi: scsi_dh_rdac: Avoid crash during rdac_bus_attach()

scsi: scsi_dh_rdac: Avoid crash during rdac_bus_attach()

The following BUG_ON() was observed during RDAC scan:

[595952.944297] kernel BUG at drivers/scsi/device_handler/scsi_dh_rdac.c:427!
[595952.951143] Internal error: Oops - BUG: 0 [#1] SMP
......
[595953.251065] Call trace:
[595953.259054]  check_ownership+0xb0/0x118
[595953.269794]  rdac_bus_attach+0x1f0/0x4b0
[595953.273787]  scsi_dh_handler_attach+0x3c/0xe8
[595953.278211]  scsi_dh_add_device+0xc4/0xe8
[595953.282291]  scsi_sysfs_add_sdev+0x8c/0x2a8
[595953.286544]  scsi_probe_and_add_lun+0x9fc/0xd00
[595953.291142]  __scsi_scan_target+0x598/0x630
[595953.295395]  scsi_scan_target+0x120/0x130
[595953.299481]  fc_user_scan+0x1a0/0x1c0 [scsi_transport_fc]
[595953.304944]  store_scan+0xb0/0x108
[595953.308420]  dev_attr_store+0x44/0x60
[595953.312160]  sysfs_kf_write+0x58/0x80
[595953.315893]  kernfs_fop_write+0xe8/0x1f0
[595953.319888]  __vfs_write+0x60/0x190
[595953.323448]  vfs_write+0xac/0x1c0
[595953.326836]  ksys_write+0x74/0xf0
[595953.330221]  __arm64_sys_write+0x24/0x30

Code is in check_ownership:

	list_for_each_entry_rcu(tmp, &h->ctlr->dh_list, node) {
		/* h->sdev should always be valid */
		BUG_ON(!tmp->sdev);
		tmp->sdev->access_state = access_state;
	}

	rdac_bus_attach
		initialize_controller
			list_add_rcu(&h->node, &h->ctlr->dh_list);
			h->sdev = sdev;

	rdac_bus_detach
		list_del_rcu(&h->node);
		h->sdev = NULL;

Fix the race between rdac_bus_attach() and rdac_bus_detach() where h->sdev
is NULL when processing the RDAC attach.

Link: https://lore.kernel.org/r/20210113063103.2698953-1-yebin10@huawei.com
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>