bcache: fix using of loop variable in memory shrink

In bch_mca_scan(), There are some confusion and logical error in the use of
loop variables. In this patch, we clarify them as:
1) nr: the number of btree nodes needs to scan, which will decrease after
we scan a btree node, and should not be less than 0;
2) i: the number of btree nodes have scanned, includes both
btree_cache_freeable and btree_cache, which should not be bigger than
btree_cache_used;
3) freed: the number of btree nodes have freed.

Signed-off-by: Tang Junhui <tang.junhui@zte.com.cn>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/btree.c

@@ -665,6 +665,7 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
 	struct btree *b, *t;
 	unsigned long i, nr = sc->nr_to_scan;
 	unsigned long freed = 0;
+	unsigned int btree_cache_used;
 
 	if (c->shrinker_disabled)
 		return SHRINK_STOP;
@@ -689,19 +690,21 @@ static unsigned long bch_mca_scan(struct shrinker *shrink,
 	nr = min_t(unsigned long, nr, mca_can_free(c));
 
 	i = 0;
+	btree_cache_used = c->btree_cache_used;
 	list_for_each_entry_safe(b, t, &c->btree_cache_freeable, list) {
-		if (freed >= nr)
-			break;
+		if (nr <= 0)
+			goto out;
 
 		if (++i > 3 &&
 		    !mca_reap(b, 0, false)) {
 			mca_data_free(b);
 			rw_unlock(true, b);
 			freed++;
 		}
+		nr--;
 	}
 
-	for (i = 0; (nr--) && i < c->btree_cache_used; i++) {
+	for (;  (nr--) && i < btree_cache_used; i++) {
 		if (list_empty(&c->btree_cache))
 			goto out;