Blktrace failed to lock reader threads on the cpu used by the corresponding

Blktrace failed to lock reader threads on the cpu used by the corresponding
writer. This resulted in stale data being consumed when blktrace accidently
read at a position that was being written to at the same time. This issue
surfaced as "bad trace magic" warnings emitted by blktrace tools.

The problem occured on an SMP System z machine. The patch fixes the issue.

Signed-off-by: Martin Peschke <mpeschke@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@carl.(none)>