selftests: icmp_redirect: IPv6 PMTU info should be cleared after redirect

[ Upstream commit 0e02bf5de46ae30074a2e1a8194a422a84482a1a ]

After redirecting, it's already a new path. So the old PMTU info should
be cleared. The IPv6 test "mtu exception plus redirect" should only
has redirect info without old PMTU.

The IPv4 test can not be changed because of legacy.

Fixes: ec810535 ("selftests: Add redirect tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>