cgroup: fix strstrip() misuse

cgroup_write_X64() and cgroup_write_string() ignore the return value of
strstrip().  it makes small inconsistent behavior.

example:
=========================
 # cd /mnt/cgroup/hoge
 # cat memory.swappiness
 60
 # echo "59 " > memory.swappiness
 # cat memory.swappiness
 59
 # echo " 58" > memory.swappiness
 bash: echo: write error: Invalid argument

This patch fixes it.

Cc: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>