bcache: panic fix for making cache device

when the nbuckets of cache device is smaller than 1024, making cache
device will trigger BUG_ON in kernel, add a condition to avoid this.

Reported-by: nitroxis <n@nxs.re>
Signed-off-by: Dongbo Cao <cdbdyx@163.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>