bpf: Change bpf_session_cookie return value to __u64 *

This reverts [1] and changes return value for bpf_session_cookie
in bpf selftests. Having long * might lead to problems on 32-bit
architectures.

Fixes: 2b8dd873 ("bpf: Make bpf_session_cookie() kfunc return long *")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240619081624.1620152-1-jolsa@kernel.org