cgroup: Add annotation for holding namespace_sem in current_cgns_cgroup_from_root()

When I initially examined the function current_cgns_cgroup_from_root(), I
was perplexed by its lack of holding cgroup_mutex. However, after Michal
explained the reason[0] to me, I realized that it already holds the
namespace_sem. I believe this intricacy could also confuse others, so it
would be advisable to include an annotation for clarification.

After we replace the cgroup_mutex with RCU read lock, if current doesn't
hold the namespace_sem, the root cgroup will be NULL. So let's add a
WARN_ON_ONCE() for it.

[0]. https://lore.kernel.org/bpf/afdnpo3jz2ic2ampud7swd6so5carkilts2mkygcaw67vbw6yh@5b5mncf7qyet

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Michal Koutny <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>