dm-crypt: Convert from tasklet to BH workqueue

The only generic interface to execute asynchronously in the BH context is
tasklet; however, it's marked deprecated and has some design flaws. To
replace tasklets, BH workqueue support was recently added. A BH workqueue
behaves similarly to regular workqueues except that the queued work items
are executed in the BH context.

This commit converts dm-crypt from tasklet to BH workqueue.  It
backfills tasklet code that was removed with commit 0a9bab39
("dm-crypt, dm-verity: disable tasklets") and tweaks to use BH
workqueue.

Like a regular workqueue, a BH workqueue allows freeing the currently
executing work item. Converting from tasklet to BH workqueue removes the
need for deferring bio_endio() again to a work item, which was buggy anyway.

I tested this lightly with "--perf-no_read_workqueue
--perf-no_write_workqueue" + some code modifications, but would really
-appreciate if someone who knows the code base better could take a look.
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/82b964f0-c2c8-a2c6-5b1f-f3145dc2c8e5@redhat.com
[snitzer: rebase ontop of commit 0a9bab39 reduced this commit's changes]
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-crypt.c

@@ -2296,7 +2296,11 @@ static void kcryptd_queue_crypt(struct dm_crypt_io *io)
 		 * irqs_disabled(): the kernel may run some IO completion from the idle thread, but
 		 * it is being executed with irqs disabled.
 		 */
-		if (!(in_hardirq() || irqs_disabled())) {
+		if (in_hardirq() || irqs_disabled()) {
+			INIT_WORK(&io->work, kcryptd_crypt);
+			queue_work(system_bh_wq, &io->work);
+			return;
+		} else {
 			kcryptd_crypt(&io->work);
 			return;
 		}