block: cleanup the flush plug helpers

Consolidate the various helpers into a single blk_flush_plug helper that
takes a plk_plug and the from_scheduler bool and switch all callsites to
call it directly.  Checks that the plug is non-NULL must be performed by
the caller, something that most already do anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20211020144119.142582-5-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/sched/core.c

@@ -6343,7 +6343,7 @@ static inline void sched_submit_work(struct task_struct *tsk)
 	 * make sure to submit it to avoid deadlocks.
 	 */
 	if (blk_needs_flush_plug(tsk))
-		blk_schedule_flush_plug(tsk);
+		blk_flush_plug(tsk->plug, true);
 }
 
 static void sched_update_worker(struct task_struct *tsk)
@@ -8354,7 +8354,8 @@ int io_schedule_prepare(void)
 	int old_iowait = current->in_iowait;
 
 	current->in_iowait = 1;
-	blk_schedule_flush_plug(current);
+	if (current->plug)
+		blk_flush_plug(current->plug, true);
 
 	return old_iowait;
 }