[XFRM] Optimize policy dumping

This change optimizes the dumping of Security policies.

1) Before this change ..
speedopolis:~# time ./ip xf pol

real    0m22.274s
user    0m0.000s
sys     0m22.269s

2) Turn off sub-policies

speedopolis:~# ./ip xf pol

real    0m13.496s
user    0m0.000s
sys     0m13.493s

i suppose the above is to be expected

3) With this change ..
speedopolis:~# time ./ip x policy

real    0m7.901s
user    0m0.008s
sys     0m7.896s

net/xfrm/xfrm_policy.c

@@ -860,33 +860,12 @@ EXPORT_SYMBOL(xfrm_policy_flush);
 int xfrm_policy_walk(u8 type, int (*func)(struct xfrm_policy *, int, int, void*),
 		     void *data)
 {
-	struct xfrm_policy *pol;
+	struct xfrm_policy *pol, *last = NULL;
 	struct hlist_node *entry;
-	int dir, count, error;
+	int dir, last_dir = 0, count, error;
 
 	read_lock_bh(&xfrm_policy_lock);
 	count = 0;
-	for (dir = 0; dir < 2*XFRM_POLICY_MAX; dir++) {
-		struct hlist_head *table = xfrm_policy_bydst[dir].table;
-		int i;
-
-		hlist_for_each_entry(pol, entry,
-				     &xfrm_policy_inexact[dir], bydst) {
-			if (pol->type == type)
-				count++;
-		}
-		for (i = xfrm_policy_bydst[dir].hmask; i >= 0; i--) {
-			hlist_for_each_entry(pol, entry, table + i, bydst) {
-				if (pol->type == type)
-					count++;
-			}
-		}
-	}
-
-	if (count == 0) {
-		error = -ENOENT;
-		goto out;
-	}
 
 	for (dir = 0; dir < 2*XFRM_POLICY_MAX; dir++) {
 		struct hlist_head *table = xfrm_policy_bydst[dir].table;
@@ -896,21 +875,37 @@ int xfrm_policy_walk(u8 type, int (*func)(struct xfrm_policy *, int, int, void*)
 				     &xfrm_policy_inexact[dir], bydst) {
 			if (pol->type != type)
 				continue;
-			error = func(pol, dir % XFRM_POLICY_MAX, --count, data);
+			if (last) {
+				error = func(last, last_dir % XFRM_POLICY_MAX,
+					     count, data);
 				if (error)
 					goto out;
 			}
+			last = pol;
+			last_dir = dir;
+			count++;
+		}
 		for (i = xfrm_policy_bydst[dir].hmask; i >= 0; i--) {
 			hlist_for_each_entry(pol, entry, table + i, bydst) {
 				if (pol->type != type)
 					continue;
-				error = func(pol, dir % XFRM_POLICY_MAX, --count, data);
+				if (last) {
+					error = func(last, last_dir % XFRM_POLICY_MAX,
+						     count, data);
 					if (error)
 						goto out;
 				}
+				last = pol;
+				last_dir = dir;
+				count++;
+			}
+		}
 	}
+	if (count == 0) {
+		error = -ENOENT;
+		goto out;
 	}
-	error = 0;
+	error = func(last, last_dir % XFRM_POLICY_MAX, 0, data);
 out:
 	read_unlock_bh(&xfrm_policy_lock);
 	return error;