net: fib_rules: Add DSCP mask attribute

net: fib_rules: Add DSCP mask attribute

Add an attribute that allows matching on DSCP with a mask. Matching on
DSCP with a mask is needed in deployments where users encode path
information into certain bits of the DSCP field.

Temporarily set the type of the attribute to 'NLA_REJECT' while support
is being added.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/20250220080525.831924-2-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>