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: Masanari Iida <standby24x7@gmail.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>