bcache: replace a mistaken IS_ERR() by IS_ERR_OR_NULL() in btree_gc_coalesce()

Commit 028ddcac ("bcache: Remove unnecessary NULL point check in
node allocations") do the following change inside btree_gc_coalesce(),

31 @@ -1340,7 +1340,7 @@ static int btree_gc_coalesce(
32         memset(new_nodes, 0, sizeof(new_nodes));
33         closure_init_stack(&cl);
34
35 -       while (nodes < GC_MERGE_NODES && !IS_ERR_OR_NULL(r[nodes].b))
36 +       while (nodes < GC_MERGE_NODES && !IS_ERR(r[nodes].b))
37                 keys += r[nodes++].keys;
38
39         blocks = btree_default_blocks(b->c) * 2 / 3;

At line 35 the original r[nodes].b is not always allocatored from
__bch_btree_node_alloc(), and possibly initialized as NULL pointer by
caller of btree_gc_coalesce(). Therefore the change at line 36 is not
correct.

This patch replaces the mistaken IS_ERR() by IS_ERR_OR_NULL() to avoid
potential issue.

Fixes: 028ddcac ("bcache: Remove unnecessary NULL point check in node allocations")
Cc:  <stable@vger.kernel.org> # 6.5+
Cc: Zheng Wang <zyytlz.wz@163.com>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20231120052503.6122-9-colyli@suse.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/btree.c

@@ -1368,7 +1368,7 @@ static int btree_gc_coalesce(struct btree *b, struct btree_op *op,
 	memset(new_nodes, 0, sizeof(new_nodes));
 	closure_init_stack(&cl);
 
-	while (nodes < GC_MERGE_NODES && !IS_ERR(r[nodes].b))
+	while (nodes < GC_MERGE_NODES && !IS_ERR_OR_NULL(r[nodes].b))
 		keys += r[nodes++].keys;
 
 	blocks = btree_default_blocks(b->c) * 2 / 3;