RX_NEXT
#define RX_NEXT(__x) (((__x)+1) & RX_RING_MOD_MASK)
#define RX_NEXT(N) (((N) + 1) & (RX_DESC_NUM-1))
#define RX_NEXT(N) (((N) + 1) & (RX_DESC_NUM_MASK))
#define RX_NEXT (0xffff)