cgroup: pin cgroup_subsys_state when opening a cgroupfs file

Previously, each file read/write operation relied on the inode
reference count pinning the cgroup and simply checked whether the
cgroup was marked dead before proceeding to invoke the per-subsystem
callback.  This was rather silly as it didn't have any synchronization
or css pinning around the check and the cgroup may be removed and all
css refs drained between the DEAD check and actual method invocation.

This patch pins the css between open() and release() so that it is
guaranteed to be alive for all file operations and remove the silly
DEAD checks from cgroup_file_read/write().

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>