zbd: add test for stressing zone locking

zbd: add test for stressing zone locking

Add a test to stress zone locking mechanism by having a large number of
threads with a small number of max_open_zones. Run 30 seconds time-based
fio under the timeout command. After 45 seconds, "timeout" kill -KILL the
fio process. If a zone lock deadlocks, fio is killed by the timeout
command, and this test fails. If not, fio runs to the end and this test
success.

Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Tested-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>