mac80211: don't attempt to rename ERR_PTR() debugfs dirs

commit 51787914 upstream.

We need to dereference the directory to get its parent to
be able to rename it, so it's clearly not safe to try to
do this with ERR_PTR() pointers. Skip in this case.

It seems that this is most likely what was causing the
report by syzbot, but I'm not entirely sure as it didn't
come with a reproducer this time.

Cc: stable@vger.kernel.org
Reported-by: syzbot+4ece1a28b8f4730547c9@syzkaller.appspotmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/mac80211/debugfs_netdev.c

@@ -838,7 +838,7 @@ void ieee80211_debugfs_rename_netdev(struct ieee80211_sub_if_data *sdata)
 
 	dir = sdata->vif.debugfs_dir;
 
-	if (!dir)
+	if (IS_ERR_OR_NULL(dir))
 		return;
 
 	sprintf(buf, "netdev:%s", sdata->name);