bpf: Fix handling maps with no BTF and non-constant offsets for the bpf_wq

bpf: Fix handling maps with no BTF and non-constant offsets for the bpf_wq

Fix handling maps with no BTF and non-constant offsets for the bpf_wq.

This de-duplicates logic with other internal structs (task_work, timer),
keeps error reporting consistent, and makes future changes to the layout
handling centralized.

Fixes: d940c9b94d7e ("bpf: add support for KF_ARG_PTR_TO_WORKQUEUE")
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251010164606.147298-1-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>