bcache: add cond_resched() in __bch_cache_cmp()

BugLink: https://bugs.launchpad.net/bugs/1840043

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>
(backported from commit d55a4ae9)
Signed-off-by: Heitor Alves de Siqueira <halves@canonical.com>
Acked-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/md/bcache/sysfs.c

@@ -820,7 +820,7 @@ SHOW(__bch_cache)
 
 	if (attr == &sysfs_priority_stats) {
 		int cmp(const void *l, const void *r)
-		{	return *((uint16_t *) r) - *((uint16_t *) l); }
+		{	cond_resched(); return *((uint16_t *) r) - *((uint16_t *) l); }
 
 		struct bucket *b;
 		size_t n = ca->sb.nbuckets, i;