Btrfs: unify error handling of btrfs_lookup_dir_item

Unify the error handling of directory item lookups using IS_ERR_OR_NULL.
No functional changes.

Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>