nl80211: fix station_info pertid memory leak

When dumping stations, memory allocated for station_info's
pertid member will leak if the nl80211 header cannot be added to
the sk_buff due to insufficient tail room.

I noticed this leak in the kmalloc-2048 cache.

Cc: stable@vger.kernel.org
Fixes: 8689c051 ("cfg80211: dynamically allocate per-tid stats for station info")
Signed-off-by: Andy Strohman <andy@uplevelsystems.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>