null_blk: use zone status for max active/open

The block layer provides special status codes when requests go beyond
the zone resource limits. Use these codes instead of the generic IOERR
for requests that exceed the max active or open limits the null_blk
device was configured with so that applications know how these special
conditions should be handled.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>