[PATCH v3 0/2] blk-mq: Allow to complete requests directly

v2…v3:
 - Align arguments with the begin of the function name
   (blk_mq_complete_request_direct).

v1…v2:
 - Drop the SCSI patch for now.
 - Make blk_mq_complete_request_direct() call the completion handler
   directly instead going through struct chain (Jens and hch might had
   the same in mind).

This series converts a part from the MMC layer which completes the
requests from kworker/ preemptible context. Its intention is to avoid
going through the softirq stack in preemptible context which would
involve the ksoftirqd in this case.