net-shapers: implement NL group operation

Allow grouping multiple leaves shaper under the given root.
The node and the leaves shapers are created, if needed, otherwise
the existing shapers are re-linked as requested.

Try hard to pre-allocated the needed resources, to avoid non
trivial H/W configuration rollbacks in case of any failure.

Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/8a721274fde18b872d1e3a61aaa916bb7b7996d3.1728460186.git.pabeni@redhat.com


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