Given a netdev that packets should be mirrored to, create a SPAN agent
and return its identifier to the caller.

The SPAN agent is reference counted, as multiple tc-mirred actions can
point to the same destination netdev.
Signed-off-by: NIdo Schimmel <idosch@mellanox.com>
Reviewed-by: NJiri Pirko <jiri@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>