net: description of dma_cookie cause make xmldocs warning

In commit 7bced397,
dma_cookie was removed from struct skbuff.
But the description of dma_cookie still exist.
So the "make xmldocs" output following warning.

Warning(.//include/linux/skbuff.h:609): Excess struct/union
/enum/typedef member 'dma_cookie' description in 'sk_buff'

Remove description of dma_cookie fix the symptom.
Signed-off-by: NMasanari Iida <standby24x7@gmail.com>
Acked-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -483,8 +483,6 @@ static inline u32 skb_mstamp_us_delta(const struct skb_mstamp *t1,
  *	@wifi_acked_valid: wifi_acked was set
  *	@wifi_acked: whether frame was acked on wifi or not
  *	@no_fcs:  Request NIC to treat last 4 bytes as Ethernet FCS
- *	@dma_cookie: a cookie to one of several possible DMA operations
- *		done by skb DMA functions
   *	@napi_id: id of the NAPI struct this skb came from
  *	@secmark: security marking
  *	@mark: Generic packet mark