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>

include/linux/netdevice.h

@@ -1708,6 +1708,7 @@ enum netdev_priv_flags {
  *	@miniq_ingress:		ingress/clsact qdisc specific data for
  *				ingress processing
  *	@ingress_queue:		XXX: need comments on this one
+ *	@nf_hooks_ingress:	netfilter hooks executed for ingress packets
  *	@broadcast:		hw bcast address
  *
  *	@rx_cpu_rmap:	CPU reverse-mapping for RX completion interrupts,