pfctl: Fix ifa_grouplookup()

Setting the length of the request got lost in r343287, which means SIOCGIFGMEMB
gives us the required length, but does not copy the names of the group members.
As a result we don't get a correct list of group members, and 'set skip on
<ifgroup>' broke.

This produced all sorts of very unexpected results, because we would end up
applying 'set skip' to unexpected interfaces.

X-MFC-with:	r343287

sbin/pfctl/pfctl_parser.c

@@ -1408,6 +1408,7 @@ ifa_grouplookup(char *ifa_name, int flags)
 		return (NULL);
 	bzero(&ifgr, sizeof(ifgr));
 	strlcpy(ifgr.ifgr_name, ifa_name, sizeof(ifgr.ifgr_name));
+	ifgr.ifgr_len = len;
 	if ((ifgr.ifgr_groups = calloc(1, len)) == NULL)
 		err(1, "calloc");
 	if (ioctl(s, SIOCGIFGMEMB, (caddr_t)&ifgr) == -1)