mac80211: Revert 'Use correct sign for mesh active path refresh'

The patch ("mac80211: Use correct sign for mesh active path
refresh.") was actually a bug.  Reverted it and improved the
explanation of how mesh path refresh works.

Signed-off-by: Javier Cardona <javier@cozybit.com>
Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/mesh_hwmp.c b/net/mac80211/mesh_hwmp.c
index 833b2f3..d28acb6 100644
--- a/net/mac80211/mesh_hwmp.c
+++ b/net/mac80211/mesh_hwmp.c
@@ -937,7 +937,7 @@
 
 	if (mpath->flags & MESH_PATH_ACTIVE) {
 		if (time_after(jiffies,
-			       mpath->exp_time +
+			       mpath->exp_time -
 			       msecs_to_jiffies(sdata->u.mesh.mshcfg.path_refresh_time)) &&
 		    !memcmp(sdata->dev->dev_addr, hdr->addr4, ETH_ALEN) &&
 		    !(mpath->flags & MESH_PATH_RESOLVING) &&