net: fib_rules: Enable DSCP selector usage

Now that both IPv4 and IPv6 support the new DSCP selector, enable user
space to configure FIB rules that make use of it by changing the policy
of the new DSCP attribute so that it accepts values in the range of [0,
63].

Use NLA_U8 rather than NLA_UINT as the field is of fixed size.

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/20240911093748.3662015-5-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>