mac80211: fix locking in ieee80211_start_ap error path

We need to hold the local->mtx to release the channel context,
as even encoded by the lockdep_assert_held() there. Fix it.

Cc: stable@vger.kernel.org
Fixes: 295b02c4 ("mac80211: Add FILS discovery support")
Reported-and-tested-by:  <syzbot+11c342e5e30e9539cabd@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20211220090836.cee3d59a1915.I36bba9b79dc2ff4d57c3c7aa30dff9a003fe8c5c@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>