mac80211: optimise synchronize_net() for sta_info_flush

There's no reason to have one synchronize_net() for each
removed station, refactor the code slightly to have just
a single synchronize_net() for all stations.

Note that this is currently useless as hostapd removes
stations one by one and this coalescing never happens.

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

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

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