ethtool: rss: support skipping contexts during dump

Applications may want to deal with dynamic RSS contexts only.
So dumping context 0 will be counter-productive for them.
Support starting the dump from a given context ID.

Alternative would be to implement a dump flag to skip just
context 0, not sure which is better...

Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/netlink/specs/ethtool.yaml

@@ -1028,6 +1028,9 @@ attribute-sets:
       -
         name: input_xfrm
         type: u32
+      -
+        name: start-context
+        type: u32
   -
     name: plca
     attributes:
@@ -1766,6 +1769,7 @@ operations:
         request:
           attributes:
             - header
+            - start-context
         reply: *rss-reply
     -
       name: plca-get-cfg