mac80211: agg-tx: don't schedule_and_wake_txq() under sta->lock

When we call ieee80211_agg_start_txq(), that will in turn call
schedule_and_wake_txq(). Called from ieee80211_stop_tx_ba_cb()
this is done under sta->lock, which leads to certain circular
lock dependencies, as reported by Chris Murphy:
https://lore.kernel.org/r/CAJCQCtSXJ5qA4bqSPY=oLRMbv-irihVvP7A2uGutEbXQVkoNaw@mail.gmail.com

In general, ieee80211_agg_start_txq() is usually not called
with sta->lock held, only in this one place. But it's always
called with sta->ampdu_mlme.mtx held, and that's therefore
clearly sufficient.

Change ieee80211_stop_tx_ba_cb() to also call it without the
sta->lock held, by factoring it out of ieee80211_remove_tid_tx()
(which is only called in this one place).

This breaks the locking chain and makes it less likely that
we'll have similar locking chain problems in the future.

Fixes: ba8c3d6f

 ("mac80211: add an intermediate software queue implementation")
Reported-by: Chris Murphy <lists@colorremedies.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20211202152554.f519884c8784.I555fef8e67d93fff3d9a304886c4a9f8b322e591@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>