mac80211: implement HS2.0 gratuitous ARP/unsolicited NA dropping

Taking the gratuitous ARP/unsolicited NA detection code from
mwifiex (but fixing it up to not have read-after-skb-end bugs),
implement the ability for userspace to request the behaviour
required by HS2.0 to drop gratuitous ARP and unsolicited NA
frames when proxy ARP service is enabled on the AP. Since this
behaviour is only mandatory for HS2.0 and may not always be
desired, make it optional - modify cfg80211/nl80211 for that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Git-commit: be9efdecf8ecdcc6d2221845482e7359b33a603b
Git-repo : git://git.kernel.org/pub/scm/linux/kernel/git/jberg/mac80211-next.git


Change-Id: I1e4083a2327c121073226aa6b75bb6b5b97cec00
CRs-fixed: 621827
[akholaif@codeaurora.org: only picked up the declaration
 and definition of cfg80211_is_gratuitous_arp_unsolicited_na()]
Signed-off-by: Ahmad Kholaif <akholaif@codeaurora.org>