mlxsw: spectrum_span: Use struct_size() to simplify allocation

Allocate the main mirroring struct and the individual structs for the
different mirroring agents in a single allocation.

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