mlxsw: spectrum_span: Add SPAN session identifier support

When packets are mirrored to the CPU, the trap identifier with which the
packets are trapped is determined according to the session identifier of
the SPAN agent performing the mirroring. Packets that are trapped for
the same logical reason (e.g., buffer drops) should use the same session
identifier.

Currently, a single session is implicitly supported (identifier 0) and
is used for packets that are mirrored to the CPU due to buffer drops
(e.g., early drop).

Subsequent patches are going to mirror packets to the CPU due to
sampling, which will require a different session identifier.

Prepare for that by making the session identifier an attribute of the
SPAN agent.

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