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

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>