um: Remove the unneeded result variable

Return the value epoll_ctl() directly instead of storing it in another
redundant variable.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Richard Weinberger <richard@nod.at>