ipv6: fib_rules: Check if rule is a default rule

As explained in commit 3c71006d

 ("ipv4: fib_rules: Check if rule is
a default rule"), drivers supporting IPv6 FIB offload need to be able to
sanitize the rules they don't support and potentially flush their
tables.

Add an IPv6 helper to check if a FIB rule is a default rule.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>