sd: fix uninitialized variable access in error handling

sd: fix uninitialized variable access in error handling

If sd_zbc_report_zones fails, the check for 'zone_blocks == 0'
later in the function accesses uninitialized data:

drivers/scsi/sd_zbc.c: In function ‘sd_zbc_read_zones’:
drivers/scsi/sd_zbc.c:520:7: error: ‘zone_blocks’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

This sets it to zero, which has the desired effect of leaving
the sd_zbc_read_zones successfully with sdkp->zone_blocks = 0.

Fixes: 89d947561077 ("sd: Implement support for ZBC devices")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Shaun Tancheff <shaun.tancheff@seagate.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>