oslib: Fix blkzoned_get_max_open_zones()

oslib: Fix blkzoned_get_max_open_zones()

When the kernel does not have the sysfs atttribute file
queue/max_open_zones, blkzoned_get_max_open_zones() returns success
without initializing the max_open_zones value to 0 to indicate to the
caller (zbd_get_max_open_zones() in zbd.c) that the device limit is
unknown. If the max_open_zones variable in zbd_get_max_open_zones() is
not already 0 (depending on the memory status), the missing
initialization in blkzoned_get_max_open_zones() can cause errors or
misbehavior as an incorrect, random, limit may be used.

Fix this by always initializing max_open_zones to 0 when the
max_open_zones sysfs attribute file does not exist.

Reported-by: Bao-Hua Li <baohua.li@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>