scsi: scsi_dh_alua: Fix possible null-ptr-deref

If alloc_workqueue fails in alua_init, it should return -ENOMEM, otherwise
it will trigger null-ptr-deref while unloading module which calls
destroy_workqueue dereference
wq->lock like this:

BUG: KASAN: null-ptr-deref in __lock_acquire+0x6b4/0x1ee0
Read of size 8 at addr 0000000000000080 by task syz-executor.0/7045

CPU: 0 PID: 7045 Comm: syz-executor.0 Tainted: G C 5.1.0+ #28
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1ubuntu1
Call Trace:
dump_stack+0xa9/0x10e
__kasan_report+0x171/0x18d
? __lock_acquire+0x6b4/0x1ee0
kasan_report+0xe/0x20
__lock_acquire+0x6b4/0x1ee0
lock_acquire+0xb4/0x1b0
__mutex_lock+0xd8/0xb90
drain_workqueue+0x25/0x290
destroy_workqueue+0x1f/0x3f0
__x64_sys_delete_module+0x244/0x330
do_syscall_64+0x72/0x2a0
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reported-by: Hulk Robot <[e-mail]>
Fixes: 03197b61c5ec ("scsi_dh_alua: Use workqueue for RTPG")
Signed-off-by: YueHaibing <[e-mail]>
Reviewed-by: Bart Van Assche <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/scsi/device_handler/scsi_dh_alua.c

···
 	int r;
 
 	kaluad_wq = alloc_workqueue("kaluad", WQ_MEM_RECLAIM, 0);
-	if (!kaluad_wq) {
-		/* Temporary failure, bypass */
-		return SCSI_DH_DEV_TEMP_BUSY;
-	}
+	if (!kaluad_wq)
+		return -ENOMEM;
 
 	r = scsi_register_device_handler(&alua_dh);
 	if (r != 0) {