iptunnel: use ll_name_to_index() for physical interface lookup

Although the cache is only initialized in do_show(), this way it is at
least consistent with ip6tunnel.

Signed-off-by: Phil Sutter <phil@nwl.cc>
diff --git a/ip/iptunnel.c b/ip/iptunnel.c
index 803bb83..a547852 100644
--- a/ip/iptunnel.c
+++ b/ip/iptunnel.c
@@ -226,7 +226,7 @@
 	}
 
 	if (medium[0]) {
-		p->link = if_nametoindex(medium);
+		p->link = ll_name_to_index(medium);
 		if (p->link == 0) {
 			fprintf(stderr, "Cannot find device \"%s\"\n", medium);
 			return -1;