net: shaper: implement introspection support

The netlink op is a simple wrapper around the device callback.

Extend the existing fetch_dev() helper adding an attribute argument
for the requested device. Reuse such helper in the newly implemented
operation.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/66eb62f22b3a5ba06ca91d01ae77515e5f447e15.1728460186.git.pabeni@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>