Cgroup id is user provided datum hence extend its return domain to
include possible error reason (similar to cgroup_get_from_fd()).

This change also fixes commit d4ccaf58 ("bpf: Introduce cgroup
iter") that would use NULL instead of proper error handling in
d4ccaf58 ("bpf: Introduce cgroup iter").

Additionally, neither of: fc_appid_store, bpf_iter_attach_cgroup,
mem_cgroup_get_from_ino (callers of cgroup_get_from_fd) is built without
CONFIG_CGROUPS (depends via CONFIG_BLK_CGROUP, direct, transitive
CONFIG_MEMCG respectively) transitive, so drop the singular definition
not needed with !CONFIG_CGROUPS.

Fixes: d4ccaf58 ("bpf: Introduce cgroup iter")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>