zbd: Fix missing mutex unlock and warnings detected with coverity

zbd: Fix missing mutex unlock and warnings detected with coverity

With max_open_zones != 0, if no candidate zone for open is found by
zbd_convert_to_open_zone(), the file zbd_info mutex as well as the
current target zone mutex must both be unlocked before returning NULL.

While at it, also assert check for min_bs != 0 where min_bs is used for
divisions to avoid division by zero warnings from coverity.

Reported-by: Bart Van Assche <bvanassche@acm.org>
Fixes: 6463db6c1d3a ("fio: fix interaction between offset/size...")
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>