netdevsim: add shared netdevsim devices

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: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>