selftests: forwarding: Use PING6 instead of ping for ipv6 multipath test

On Debian jessie ping can not handle IPv6 addresses so the command
fails. Use PING6 which is set to ping6.
Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/forwarding/router_multipath.sh

@@ -249,7 +249,7 @@ multipath6_test()
 
        # Generate 16384 echo requests, each with a random flow label.
        for _ in $(seq 1 16384); do
-	       ip vrf exec vrf-h1 ping 2001:db8:2::2 -F 0 -c 1 -q &> /dev/null
+	       ip vrf exec vrf-h1 $PING6 2001:db8:2::2 -F 0 -c 1 -q &> /dev/null
        done
 
        t1_rp12=$(link_stats_tx_packets_get $rp12)