zbd: introduce per job maximum open zones limit

zbd: introduce per job maximum open zones limit

It is not possible to maintain sustained per-thread iodepth in ZBD mode.
The way code is written, "max_open_zones" acts as a global limit, and
once one or few threads open all "max_open_zones" zones, other threads
can't open anything and _exit_ prematurely.

This config is guaranteed to make equal number of zone resets/IO now:
each thread generates identical pattern and doesn't intersect with other
threads:

	zonemode=zbd
	zonesize=...
	rw=write

	numjobs=N
	offset_increment=M*zonesize

	[j]
	size=M*zonesize

Patch introduces "job_max_open_zones" which is per-thread/process limit.
"max_open_zones" remains per file/device limit. Both limits are checked
for each open zone so one thread can't kick out others.

Signed-off-by: Alexey Dobriyan (SK hynix) <adobriyan@gmail.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>