Add missing kernel-doc notation to sk_buff:

Warning(linux-2.6.27-rc1-git2//include/linux/skbuff.h:345): No description found for parameter 'do_not_encrypt'
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>