btrfs: replace BUG_ON() with error return in cache_save_setup()

In cache_save_setup(), if create_free_space_inode() succeeds but the
subsequent lookup_free_space_inode() still fails on retry, the
BUG_ON(retries) will crash the kernel. This can happen due to I/O
errors or transient failures, not just programming bugs.

Replace the BUG_ON with proper error handling that returns the original
error code through the existing cleanup path. The callers already handle
this gracefully: disk_cache_state defaults to BTRFS_DC_ERROR, so the
space cache simply won't be written for that block group.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Teng Liu <27rabbitlt@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: 27rabbitlt

fs/btrfs/block-group.c

@@ -3381,7 +3381,13 @@ static void cache_save_setup(struct btrfs_block_group *block_group,
 	}
 
 	if (IS_ERR(inode)) {
-		BUG_ON(retries);
+		if (retries) {
+			ret = PTR_ERR(inode);
+			btrfs_err(fs_info,
+				  "failed to lookup free space inode after creation for block group %llu: %d",
+				  block_group->start, ret);
+			goto out_free;
+		}
 		retries++;
 
 		if (block_group->ro)