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>