xfrm: optimise the use of walk list header in xfrm_policy/state_walk

The walk from input is the list header, and marked as dead, and will
be skipped in loop.

list_first_entry() can be used to return the true usable value from
walk if walk is not empty
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_policy.c

@@ -1004,7 +1004,9 @@ int xfrm_policy_walk(struct net *net, struct xfrm_policy_walk *walk,
 	if (list_empty(&walk->walk.all))
 		x = list_first_entry(&net->xfrm.policy_all, struct xfrm_policy_walk_entry, all);
 	else
-		x = list_entry(&walk->walk.all, struct xfrm_policy_walk_entry, all);
+		x = list_first_entry(&walk->walk.all,
+				     struct xfrm_policy_walk_entry, all);
+
 	list_for_each_entry_from(x, &net->xfrm.policy_all, all) {
 		if (x->dead)
 			continue;

net/xfrm/xfrm_state.c

@@ -1626,7 +1626,7 @@ int xfrm_state_walk(struct net *net, struct xfrm_state_walk *walk,
 	if (list_empty(&walk->all))
 		x = list_first_entry(&net->xfrm.state_all, struct xfrm_state_walk, all);
 	else
-		x = list_entry(&walk->all, struct xfrm_state_walk, all);
+		x = list_first_entry(&walk->all, struct xfrm_state_walk, all);
 	list_for_each_entry_from(x, &net->xfrm.state_all, all) {
 		if (x->state == XFRM_STATE_DEAD)
 			continue;