scsi: sd_zbc: Remove set but not used variable 'buflen'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/scsi/sd_zbc.c: In function 'sd_zbc_check_zones':
drivers/scsi/sd_zbc.c:341:9: warning:
 variable 'buflen' set but not used [-Wunused-but-set-variable]

It is not used since commit d9dd7308 ("block: Enhance
blk_revalidate_disk_zones()")

Reported-by: Hulk Robot <hulkci@huawei.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>