bcache: fix inaccurate result of unused buckets

To get the amount of unused buckets in sysfs_priority_stats, the code
count the buckets which GC_SECTORS_USED is zero. It's correct and should
not be overwritten by the count of buckets which prio is zero.

Signed-off-by: Guoju Fang <fangguoju@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/sysfs.c

@@ -996,8 +996,6 @@ SHOW(__bch_cache)
 		       !cached[n - 1])
 			--n;
 
-		unused = ca->sb.nbuckets - n;
-
 		while (cached < p + n &&
 		       *cached == BTREE_PRIO)
 			cached++, n--;