vswitchd: Consistently use size_t for action lengths.

Currently the type of the datapath action length is mixture of
size_t and unsigned int.  However, size_t is really defined as an
unsigned long, which causes the build to fail on 64-bit platforms.
This consistently uses size_t.

lib/odp-util.c

@@ -171,7 +171,7 @@ format_odp_action(struct ds *ds, const struct nlattr *a)
 
 void
 format_odp_actions(struct ds *ds, const struct nlattr *actions,
-                   unsigned int actions_len)
+                   size_t actions_len)
 {
     if (actions_len) {
         const struct nlattr *a;