bpf, cgroup: Return ENOLINK for auto-detached links on update

Failure to update a bpf_link because it has been auto-detached by a dying
cgroup currently results in EINVAL error, even though the arguments passed
to bpf() syscall are not wrong.

bpf_links attaching to netns in this case will return ENOLINK, which
carries the message that the link is no longer attached to anything.

Change cgroup bpf_links to do the same to keep the uAPI errors consistent.

Fixes: 0c991ebc ("bpf: Implement bpf_prog replacement for an active bpf_cgroup_link")
Suggested-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200531082846.2117903-6-jakub@cloudflare.com