netfilter: Document ingress hook

Amend kerneldoc of struct net_device to fix a "make htmldocs" warning:

include/linux/netdevice.h:2045: warning: Function parameter or member 'nf_hooks_ingress' not described in 'net_device'

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>