mac80211: Fix sta_mtx unlocking on insert STA failure path

Commit 34e895075e21be3e21e71d6317440d1ee7969ad0 introduced sta_mtx
locking into sta_info_insert() (now sta_info_insert_rcu), but forgot
to unlock this mutex on one of the error paths. Fix this by adding
the missing mutex_unlock() call for the case where STA insert fails
due to an entry existing already. This may happen at least in AP mode
when a STA roams between two BSSes (vifs).

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
index 211c475..56422d8 100644
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -434,6 +434,7 @@
 	/* check if STA exists already */
 	if (sta_info_get_bss(sdata, sta->sta.addr)) {
 		spin_unlock_irqrestore(&local->sta_lock, flags);
+		mutex_unlock(&local->sta_mtx);
 		rcu_read_lock();
 		err = -EEXIST;
 		goto out_free;