selftests: fib_rule_tests: Fix icmp proto with ipv6

A recent commit returns an error if icmp is used as the ip-proto for
IPv6 fib rules. Update fib_rule_tests to send ipv6-icmp instead of icmp.

Fixes: 5e1a99ea ("ipv4: Add ICMPv6 support when parse route ipproto")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>