LU-12946 kernel: fix to handle BLK_MQ_RQ_QUEUE_DEV_BUSY event

LU-12946 kernel: fix to handle BLK_MQ_RQ_QUEUE_DEV_BUSY event

It looks like what's happening is when dm_dispatch_clone_request
dispatches the "clone" I/O request to the underlying (real) device
from the multipath device, the scsi driver can (often under load)
return BLK_MQ_RQ_QUEUE_DEV_BUSY. dm_dispatch_clone_request doesn't
have that as an exception the way it does BLK_MQ_RQ_QUEUE_BUSY and
so it calls dm_complete_request which propagates
the BLK_MQ_RQ_QUEUE_DEV_BUSY error code up the stack resulting
in multipath_end_io calling fail_path and failing the path because
there is an error value set.

Lustre-change: https://review.whamcloud.com/36699
Lustre-commit: 5c8b1e87a97bbe7b05f0b8325e98c16a0de1ff4c

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: If17ea5b3ab33a89a17d49e5dfb2e9f9f19371564
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36868
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>