netkit: Add add netkit {head,tail}room to rt_link.yaml

Add netkit {head,tail}room attribute support to the rt_link.yaml spec file.

Example:

  # ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/rt_link.yaml \
   --do getlink --json '{"ifname": "nk0"}' --output-json | jq
  [...]
  "linkinfo": {
    "kind": "netkit",
    "data": {
    }
  },
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Nikolay Aleksandrov <razor@blackwall.org>

Author: borkmann

Documentation/netlink/specs/rt_link.yaml

@@ -2166,6 +2166,12 @@ attribute-sets:
         name: peer-scrub
         type: u32
         enum: netkit-scrub
+      -
+        name: headroom
+        type: u16
+      -
+        name: tailroom
+        type: u16
 
 sub-messages:
   -