ipv6: fib_rules: Add DSCP selector support

ipv6: fib_rules: Add DSCP selector support

Implement support for the new DSCP selector that allows IPv6 FIB rules
to match on the entire DSCP field. This is done despite the fact that
the above can be achieved using the existing TOS selector, so that user
space program will be able to work with IPv4 and IPv6 rules in the same
way.

Differentiate between both selectors by adding a new bit in the IPv6 FIB
rule structure that is only set when the 'FRA_DSCP' attribute is
specified by user space. Reject rules that use both selectors.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20240911093748.3662015-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>