bpf: Move cgroup iterator helpers to bpf.h

[ Upstream commit 9621e60f ]

Move them into bpf.h given we also need them in core code.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20250730234733.530041-3-daniel@iogearbox.net


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Stable-dep-of: abad3d0b ("bpf: Fix oob access in cgroup local storage")
Signed-off-by: Sasha Levin <sashal@kernel.org>