selftests: xsk: Disable IPv6 on VETH1

This change fixes flakiness in the BIDIRECTIONAL test:

    # [is_pkt_valid] expected length [60], got length [90]
    not ok 1 FAIL: SKB BUSY-POLL BIDIRECTIONAL

When IPv6 is enabled, the interface will periodically send MLDv1 and
MLDv2 packets. These packets can cause the BIDIRECTIONAL test to fail
since it uses VETH0 for RX.

For other tests, this was not a problem since they only receive on VETH1
and IPv6 was already disabled on VETH0.

Fixes: a8905257 ("selftests/bpf: Xsk selftests framework")
Signed-off-by: Kal Conley <kal.conley@dectris.com>
Link: https://lore.kernel.org/r/20230405082905.6303-1-kal.conley@dectris.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/test_xsk.sh

@@ -116,6 +116,7 @@ setup_vethPairs() {
 	ip link add ${VETH0} numtxqueues 4 numrxqueues 4 type veth peer name ${VETH1} numtxqueues 4 numrxqueues 4
 	if [ -f /proc/net/if_inet6 ]; then
 		echo 1 > /proc/sys/net/ipv6/conf/${VETH0}/disable_ipv6
+		echo 1 > /proc/sys/net/ipv6/conf/${VETH1}/disable_ipv6
 	fi
 	if [[ $verbose -eq 1 ]]; then
 	        echo "setting up ${VETH1}"