cpuset: drop spurious retval assignment in proc_cpuset_show()

proc_cpuset_show() has a spurious -EINVAL assignment which does
nothing.  Remove it.

This patch doesn't make any functional difference.

tj: Rewrote patch description.

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