mlxsw: spectrum_span: Add APIs to get / put a SPAN agent

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: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>