Use au->lock exclusively for locking in async I/O.

Use au->lock exclusively for locking in async I/O.

2020-02-18  Thomas Koenig  <tkoenig@gcc.gnu.org>

	Backport from trunk
	PR fortran/93599
	* io/async.c (destroy_adv_cond): Do not destroy lock.
	(async_io): Make sure au->lock is locked for finishing of thread.
	Do not lock/unlock around signalling emptysignal. Unlock au->lock
	before return.
	(init_adv_cond): Do not initialize lock.
	(enqueue_transfer): Unlock after signal.
	(enqueue_done_id): Likewise.
	(enqueue_done): Likewise.
	(enqueue_close): Likewise.
	(enqueue_data_transfer): Likewise.
	(async_wait_id): Do not lock/unlock around signalling au->work.
	(async_wait): Unlock after signal.
	* io/async.h (SIGNAL): Add comment about needed au->lock.
	Remove locking/unlocking of advcond->lock.
	(WAIT_SIGNAL_MUTEX): Add comment. Remove locking/unlocking of
	advcond->lock.  Unlock mutex only at the end.  Loop on
	__ghread_cond_wait returning zero.
	(REVOKE_SIGNAL): Add comment. Remove locking/unlocking of
	advcond->lock.
	(struct adv_cond): Remove mutex from struct.