cfg80211: check dev_set_name() return value

syzbot reported a warning from rfkill_alloc(), and after a while
I think that the reason is that it was doing fault injection and
the dev_set_name() failed, leaving the name NULL, and we didn't
check the return value and got to rfkill_alloc() with a NULL name.
Since we really don't want a NULL name, we ought to check the
return value.

Fixes: fb28ad35906a ("net: struct device - replace bus_id with dev_name(), dev_set_name()")
Reported-by: syzbot+1ddfb3357e1d7bb5b5d3@syzkaller.appspotmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/core.c

@@ -439,6 +439,8 @@ struct wiphy *wiphy_new_nm(const struct cfg80211_ops *ops, int sizeof_priv,
 		if (rv)
 			goto use_default_name;
 	} else {
+		int rv;
+
 use_default_name:
 		/* NOTE:  This is *probably* safe w/out holding rtnl because of
 		 * the restrictions on phy names.  Probably this call could
@@ -446,7 +448,11 @@ use_default_name:
 		 * phyX.  But, might should add some locking and check return
 		 * value, and use a different name if this one exists?
 		 */
-		dev_set_name(&rdev->wiphy.dev, PHY_NAME "%d", rdev->wiphy_idx);
+		rv = dev_set_name(&rdev->wiphy.dev, PHY_NAME "%d", rdev->wiphy_idx);
+		if (rv < 0) {
+			kfree(rdev);
+			return NULL;
+		}
 	}
 
 	INIT_LIST_HEAD(&rdev->wiphy.wdev_list);