memcg: remove redundant code in mem_cgroup_force_empty_write()

memcg: remove redundant code in mem_cgroup_force_empty_write()

vfs guarantees the cgroup won't be destroyed, so it's redundant to get a
css reference.

Signed-off-by: Li Zefan <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>