Provide infrastructure to create and destroy representors of a given type.

Parts based on work by Bert van Leeuwen, Benjamin LaHaise,
and Jakub Kicinski.
Signed-off-by: NSimon Horman <simon.horman@netronome.com>
Reviewed-by: NJakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>