netlabel: remove impossible return value in netlbl_bitmap_walk

netlabel: remove impossible return value in netlbl_bitmap_walk

Since commit 446fda4f2682 ("[NetLabel]: CIPSOv4 engine"), *bitmap_walk
function only returns -1. Nearly 18 years have passed, -2 scenes never
come up, so there's no need to consider it.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Link: https://lore.kernel.org/r/20240227093604.3574241-1-shaozhengchao@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>