Factor out sharable netdevsim sub-object and use IFLA_LINK to link
netdevsims together at creation time.  Sharable object will have
its own DebugFS directory.
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: NQuentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>