mlxsw: spectrum_span: Use 'refcount_t' for reference counting

'refcount_t' is very useful for catching over/under flows. Convert the
SPAN agent objects to use it instead of 'int' for their reference count.

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