ip: Publish functions for stats formatting

ip: Publish functions for stats formatting

Formatting struct rtnl_link_stats64 will be useful outside of iplink.c as
well. Extract from __print_link_stats() a new function, print_stats64(),
make it non-static and publish in the header file.

Additionally, publish the helper size_columns(), which will be useful for
formatting the new struct rtnl_hw_stats64.

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