libnetlink: Add filtering to rtnl_statsdump_req_filter()

libnetlink: Add filtering to rtnl_statsdump_req_filter()

A number of functions in the rtnl_*_req family accept a caller-provided
callback to set up arbitrary filtering. rtnl_statsdump_req_filter()
currently only allows setting a field in the IFSM header, not custom
attributes. So far these were not necessary, but with introduction of more
detailed filtering settings, the callback becomes necessary.

To that end, add a filter_fn and filter_data arguments to the function.
Unlike the other filters, this one is typed to expect an IFSM pointer, to
permit tweaking the header itself as well.

Pass NULLs in the existing callers.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>