netlink: specs: tc: add qdisc dump to TC spec

Hook TC qdisc dump in the TC qdisc get, it only supported doit
until now and dumping will be used by the sample code.

Reviewed-by: Donald Hunter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/netlink/specs/tc.yaml

···
       doc: Get / dump tc qdisc information.
       attribute-set: attrs
       fixed-header: tcmsg
-      do:
+      do: &getqdisc-do
         request:
           value: 38
           attributes:
···
             - chain
             - ingress-block
             - egress-block
+      dump: *getqdisc-do
     -
       name: newtclass
       doc: Get / dump tc traffic class information.