mac80211: Move call to mpp_path_lookup inside RCU-read section

PROVE_RCU caught that one:

[  431.214070] ===================================================
[  431.215341] [ INFO: suspicious rcu_dereference_check() usage. ]
[  431.215674] ---------------------------------------------------
[  431.216043] net/mac80211/mesh_pathtbl.c:184 invoked
rcu_dereference_check() without protection!
Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/tx.c

@@ -1751,6 +1751,7 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 			ret = NETDEV_TX_OK;
 			goto fail;
 		}
+		rcu_read_lock();
 		if (!is_multicast_ether_addr(skb->data))
 			mppath = mpp_path_lookup(skb->data, sdata);
 
@@ -1765,13 +1766,13 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 		    !(mppath && compare_ether_addr(mppath->mpp, skb->data))) {
 			hdrlen = ieee80211_fill_mesh_addresses(&hdr, &fc,
 					skb->data, skb->data + ETH_ALEN);
+			rcu_read_unlock();
 			meshhdrlen = ieee80211_new_mesh_header(&mesh_hdr,
 					sdata, NULL, NULL);
 		} else {
 			int is_mesh_mcast = 1;
 			const u8 *mesh_da;
 
-			rcu_read_lock();
 			if (is_multicast_ether_addr(skb->data))
 				/* DA TA mSA AE:SA */
 				mesh_da = skb->data;