mac80211: add interface list lock

Using only the RTNL has a number of problems, most notably that
ieee80211_iterate_active_interfaces() and other interface list
traversals cannot be done from the internal workqueue because it
needs to be flushed under the RTNL.

This patch introduces a new mutex that protects the interface list
against modifications. A more detailed explanation is part of the
code change.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/ieee80211_i.h b/net/mac80211/ieee80211_i.h
index 927cbde..eaf3603 100644
--- a/net/mac80211/ieee80211_i.h
+++ b/net/mac80211/ieee80211_i.h
@@ -643,7 +643,9 @@
 	struct crypto_blkcipher *wep_rx_tfm;
 	u32 wep_iv;
 
+	/* see iface.c */
 	struct list_head interfaces;
+	struct mutex iflist_mtx;
 
 	/*
 	 * Key lock, protects sdata's key_list and sta_info's