Fix potential scheduling while atomic in mesh_path_add.

Calling synchronize_rcu() under write-lock-ed pathtbl_resize_lock may
result in this warning (and other side effects).

It looks safe just dropping this lock before calling synchronize_rcu.
Signed-off-by: NPavel Emelyanov <xemul@openvz.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

net/mac80211/mesh_pathtbl.c

@@ -220,9 +220,10 @@ int mesh_path_add(u8 *dst, struct net_device *dev)
 			return -ENOMEM;
 		}
 		rcu_assign_pointer(mesh_paths, newtbl);
+		write_unlock(&pathtbl_resize_lock);
+
 		synchronize_rcu();
 		mesh_table_free(oldtbl, false);
-		write_unlock(&pathtbl_resize_lock);
 	}
 endadd2:
 	return err;