mac80211: batch key free synchronize_net()

Instead of calling synchronize_net() for every key
on an interface or when a station is removed, do it
only once for all keys in both of these cases.

As a side-effect, removing station keys now always
calls synchronize_net() even if there are no keys,
which fixes an issue with station removal happening
in the driver while the station could still be used
for TX.

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

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

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

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