dm rq: do not update rq partially in each ending bio

We don't need to update the original dm request partially when ending
each cloned bio: just update original dm request once when the whole
cloned request is finished.  This still allows full support for partial
completion because a new 'completed' counter accounts for incremental
progress as the clone bios complete.

Partial request update can be a bit expensive, so we should try to avoid
it, especially because it is run in softirq context.

Avoiding all the partial request updates fixes both hard lockup and
soft lockups that were easily reproduced while running Laurence's
test[1] on IB/SRP.

BTW, after d4acf3650c7c ("block: Make blk_mq_delay_kick_requeue_list()
rerun the queue at a quiet time"), we need to make the test more
aggressive for reproducing the lockup:

	1) run hammer_write.sh 32 or 64 concurrently.
	2) write 8M each time

[1] https://marc.info/?l=linux-block&m=150220185510245&w=2

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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