blktrace: Show requests without sector

Currently, blktrace will not show requests that don't have any data as
rq->__sector is initialized to -1 which is out of device range and thus
discarded by act_log_check(). This is most notably the case for cache
flush requests sent to the device. Fix the problem by making
blk_rq_trace_sector() return 0 for requests without initialized sector.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>