blktrace: don't stop tracer if not setup trace successfully

if we run blktrace on same device twice, the second time will failed
to ioctl(BLKTRACESETUP), then it will call __stop_tracer, which lead
the first blktrace failed to access debugfs entries. So this patch add
a check to handle this case, to avoid stop tracer uncondionally.

Signed-off-by: weiping zhang <zhangweiping@didichuxing.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>