bcache: add cond_resched() in __bch_cache_cmp()

Read /sys/fs/bcache/<uuid>/cacheN/priority_stats can take very long
time with huge cache after long run.

Signed-off-by: Shile Zhang <shile.zhang@linux.alibaba.com>
Tested-by: Heitor Alves de Siqueira <halves@canonical.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>