block: ataflop: more blk-mq refactoring fixes

As it turns out, my earlier patch in commit 86d46fdaa12a (block:
ataflop: fix breakage introduced at blk-mq refactoring) was
incomplete. This patch fixes any remaining issues found during
more testing and code review.

Requests exceeding 4 k are handled in 4k segments but
__blk_mq_end_request() is never called on these (still
sectors outstanding on the request). With redo_fd_request()
removed, there is no provision to kick off processing of the
next segment, causing requests exceeding 4k to hang. (By
setting /sys/block/fd0/queue/max_sectors_k <= 4 as workaround,
this behaviour can be avoided).

Instead of reintroducing redo_fd_request(), requeue the remainder
of the request by calling blk_mq_requeue_request() on incomplete
requests (i.e. when blk_update_request() still returns true), and
rely on the block layer to queue the residual as new request.

Both error handling and formatting needs to release the
ST-DMA lock, so call finish_fdc() on these (this was previously
handled by redo_fd_request()). finish_fdc() may be called
legitimately without the ST-DMA lock held - make sure we only
release the lock if we actually held it. In a similar way,
early exit due to errors in ataflop_queue_rq() must release
the lock.

After minor errors, fd_error sets up to recalibrate the drive
but never re-runs the current operation (another task handled by
redo_fd_request() before). Call do_fd_action() to get the next
steps (seek, retry read/write) underway.

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Fixes: 6ec3938cff95f (ataflop: convert to blk-mq)
CC: linux-block@vger.kernel.org
Link: https://lore.kernel.org/r/20211024002013.9332-1-schmitzmic@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>