skbuff: fix missing kernel-doc notation

Add kernel-doc notation for ndisc_nodetype:

Warning(linux-2.6.25-git2//include/linux/skbuff.h:340): No description found for parameter 'ndisc_nodetype'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>