netlink: specs: Add FIB rule flow label attributes

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

# ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
--do newrule \
--json '{"family": 10, "flowlabel": 1, "flowlabel-mask": 1, "action": 1, "table": 1}'
None
$ ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_rule.yaml \
--dump getrule --json '{"family": 10}' --output-json \
| jq '.[] | select(.flowlabel == "0x1")'
{
"table": 1,
"suppress-prefixlen": "0xffffffff",
"protocol": 0,
"priority": 32765,
"flowlabel": "0x1",
"flowlabel-mask": "0x1",
"family": 10,
"dst-len": 0,
"src-len": 0,
"tos": 0,
"action": "to-tbl",
"flags": 0
}

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_rule.yaml

···
       -
         name: dscp
         type: u8
+      -
+        name: flowlabel
+        type: u32
+        byte-order: big-endian
+        display-hint: hex
+      -
+        name: flowlabel-mask
+        type: u32
+        byte-order: big-endian
+        display-hint: hex
 
 operations:
   enum-model: directional
···
             - sport-range
             - dport-range
             - dscp
+            - flowlabel
+            - flowlabel-mask
     -
       name: newrule-ntf
       doc: Notify a rule creation