ipv6: fib_rules: Add DSCP mask matching

ipv6: fib_rules: Add DSCP mask matching

Extend IPv6 FIB rules to match on DSCP using a mask. Unlike IPv4, also
initialize the DSCP mask when a non-zero 'tos' is specified as there is
no difference in matching between 'tos' and 'dscp'. As a side effect,
this makes it possible to match on 'dscp 0', like in IPv4.

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/20250220080525.831924-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>