gdrom: dequeue in-flight request

gdrom already dequeues and fully completes requests on normal path and
the error paths can be easily converted to do so too.  Clean it up and
dequeue requests on error paths too.

While at it remove superflous blk_fs_request() && !blk_rq_sectors()
condition check.

[ Impact: dequeue in-flight request, cleanup ]

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>