mac80211: make scan_sdata pointer usable with RCU

Making the scan_sdata pointer usable with RCU makes
it possible to dereference it in the RX path to see
if a received frame actually matches the interface
that is scanning. This is just preparations, making
the pointer __rcu.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc net/mac80211/ieee80211_i.h
Simple merge

diff --cc net/mac80211/iface.c
Simple merge

diff --cc net/mac80211/scan.c
Simple merge