UPSTREAM: mac80211: rework locking for txq scheduling / airtime fairness

Holding the lock around the entire duration of tx scheduling can create
some nasty lock contention, especially when processing airtime information
from the tx status or the rx path.
Improve locking by only holding the active_txq_lock for lookups / scheduling
list modifications.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Hiland-Jrgensen <toke@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

BUG=b:131197056
TEST=Wifi stress and latency under load test
(cherry picked from commit 5b989c18)

Signed-off-by: Kan Yan <kyan@google.com>
Change-Id: If9695edb86f92cb6f629f58e5ce87f6a20c5deff
Reviewed-on: https://chromium-review.googlesource.com/1636902


Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Julan Hsu <julanhsu@google.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Kan Yan <kyan@chromium.org>