zbd: Fix potential deadlock on read operations

zbd: Fix potential deadlock on read operations

For read-only workloads, zbd_find_zone() has a similar zone locking
behavior as for write IOs: zones to be read are locked when an IO is
prepared and unlocked when the IO completes. With an asynchronous IO
engine, this can create deadlocks if 2 threads are trying to read the
same 2 zones. For instance, if thread A already has a lock on zone 1
and is waiting for a lock on zone 2 while thread B already has a lock
on zone 2 and waiting for a lock on zone 1.

The fix is similar to previous fixes for this potential deadlock,
namely, use zone_lock() instead of directly calling pthread_mutex_lock()
to ensure that a thread issues the IOs it already has prepared if it
encounters a locked zone, doing so ensuring forward progress.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>