netlink: specs: Add FIB rule DSCP mask attribute

Add new DSCP mask attribute to the spec. Example:

 # ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
	 --do newrule \
	 --json '{"family": 2, "dscp": 10, "dscp-mask": 63, "action": 1, "table": 1}'
 None
 $ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
	 --dump getrule --json '{"family": 2}' --output-json | jq '.[]'
 [...]
 {
   "table": 1,
   "suppress-prefixlen": "0xffffffff",
   "protocol": 0,
   "priority": 32765,
   "dscp": 10,
   "dscp-mask": "0x3f",
   "family": 2,
   "dst-len": 0,
   "src-len": 0,
   "tos": 0,
   "action": "to-tbl",
   "flags": 0
 }
 [...]

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/20250220080525.831924-6-idosch@nvidia.com


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