netlink: specs: Add route flow label attribute

Add the new flow label attribute to the spec. Example:

# ip link add name dummy1 up type dummy
# ip -6 route add default table 254 dev dummy1
# ip -6 route add default table 10 dev dummy1
# ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
--do newrule \
--json '{"family": 10, "priority": 1, "flowlabel": 10, "flowlabel-mask": 255, "action": 1, "table": 10}'
None
$ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_route.yaml \
--do getroute \
--json '{"rtm-family": 10, "rta-flowlabel": 1}' --output-json \
| jq '.["rta-table"]'
254
$ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_route.yaml \
--do getroute \
--json '{"rtm-family": 10, "rta-flowlabel": 10}' --output-json \
| jq '.["rta-table"]'
10

Reviewed-by: Petr Machata <[e-mail]>
Signed-off-by: Ido Schimmel <[e-mail]>
Reviewed-by: Guillaume Nault <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

Documentation/netlink/specs/rt_route.yaml

···
       -
         name: rta-nh-id
         type: u32
+      -
+        name: rta-flowlabel
+        type: u32
+        byte-order: big-endian
+        display-hint: hex
   -
     name: rta-metrics
     attributes:
···
             - rta-dport
             - rta-mark
             - rta-uid
+            - rta-flowlabel
         reply:
           value: 24
           attributes: &all-route-attrs
···
             - rta-sport
             - rta-dport
             - rta-nh-id
+            - rta-flowlabel
       dump:
         request:
           value: 26