cgroup: Honor caller's cgroup NS when resolving path

cgroup: Honor caller's cgroup NS when resolving path

[ Upstream commit 74e4b956eb1cac0e4c10c240339b1bbfbc9a4c48 ]

cgroup_get_from_path() is not widely used function. Its callers presume
the path is resolved under cgroup namespace. (There is one caller
currently and resolving in init NS won't make harm (netfilter). However,
future users may be subject to different effects when resolving
globally.)
Since, there's currently no use for the global resolution, modify the
existing function to take cgroup NS into account.

Fixes: a79a908fd2b0 ("cgroup: introduce cgroup namespaces")
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>