lwtunnel: fix argument parsing

Currently parse_encap_ip() does not update correctly argv/argc;
if multiple lwtunnel arguments are provided, the parsing fails after
the first one, i.e.

 ip route add 172.16.101.0/24 dev vxlan1 encap ip id 42 dst 192.168.255.1

fails with:

 Error: either "to" is duplicate, or "dst" is a garbage.

This commit addresses the issue, stepping to next argument at each iteration
of the parsing loop.

Fixes: 1e529305 ("lwtunnel: Add encapsulation support to ip route")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

ip/iproute_lwtunnel.c

@@ -201,10 +201,14 @@ static int parse_encap_ip(struct rtattr *rta, size_t len, int *argcp, char ***ar
 		} else {
 			break;
 		}
+		argc--; argv++;
 	}
 
-	*argcp = argc;
-	*argvp = argv;
+	/* argv is currently the first unparsed argument,
+	 * but the lwt_parse_encap() caller will move to the next,
+	 * so step back */
+	*argcp = argc + 1;
+	*argvp = argv - 1;
 
 	return 0;
 }