zbd: return ENOMEM if zone buffer allocation fails

zbd: return ENOMEM if zone buffer allocation fails

parse_zone_info() function tries to allocate a buffer of
ZBD_REPORT_MAX_ZONES zone descriptors and exits if this allocation
fails. The problem is that it returns 0 error code in this case and
the caller may interpret this as the success.

Just return ENOMEM if we can't allocate that buffer.

Signed-off-by: Dmitry Fomichev <dmitry.fomichev@wdc.com>
Reviewed-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>