bcachefs: fix the count of nr_freed_pcpu after changing bc->freed_nonpcpu list

When allocating bkey_cached from bc->freed_pcpu list, it missed
decreasing the count of nr_freed_pcpu which would cause the mismatch
between the value of nr_freed_pcpu and the list items. This problem
also exists in moving new bkey_cached to bc->freed_pcpu list.
If these happened, the bug info may appear in
bch2_fs_btree_key_cache_exit by the follow code:

   BUG_ON(list_count_nodes(&bc->freed_pcpu) != bc->nr_freed_pcpu);
   BUG_ON(list_count_nodes(&bc->freed_nonpcpu) != bc->nr_freed_nonpcpu);

Fixes: c65c13f0 ("bcachefs: Run btree key cache shrinker less aggressively")
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_key_cache.c

@@ -169,6 +169,7 @@ static void bkey_cached_move_to_freelist(struct btree_key_cache *bc,
 	} else {
 		mutex_lock(&bc->lock);
 		list_move_tail(&ck->list, &bc->freed_pcpu);
+		bc->nr_freed_pcpu++;
 		mutex_unlock(&bc->lock);
 	}
 }
@@ -245,6 +246,7 @@ bkey_cached_alloc(struct btree_trans *trans, struct btree_path *path,
 		if (!list_empty(&bc->freed_pcpu)) {
 			ck = list_last_entry(&bc->freed_pcpu, struct bkey_cached, list);
 			list_del_init(&ck->list);
+			bc->nr_freed_pcpu--;
 		}
 		mutex_unlock(&bc->lock);
 	}