target: Fix alua_access_state attribute OOPs for un-configured devices

This patch fixes a OOPs where an attempt to write to the per-device
alua_access_state configfs attribute at:

/sys/kernel/config/target/core/$HBA/$DEV/alua/$TG_PT_GP/alua_access_state

results in an NULL pointer dereference when the backend device has not
yet been configured.

This patch adds an explicit check for DF_CONFIGURED, and fails with
-ENODEV to avoid this case.

Reported-by: Chris Boot <[e-mail]>
Reported-by: Philip Gaw <[e-mail]>
Cc: Chris Boot <[e-mail]>
Cc: Philip Gaw <[e-mail]>
Cc: [e-mail] # 3.8+
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/target_core_configfs.c

···
 			" tg_pt_gp ID: %hu\n", tg_pt_gp->tg_pt_gp_valid_id);
 		return -EINVAL;
 	}
+	if (!(dev->dev_flags & DF_CONFIGURED)) {
+		pr_err("Unable to set alua_access_state while device is"
+		       " not configured\n");
+		return -ENODEV;
+	}
 
 	ret = kstrtoul(page, 0, &tmp);
 	if (ret < 0) {