proc: fix reported unit for RLIMIT_CPU

/proc/$pid/limits should show RLIMIT_CPU as seconds, which is the unit
used in kernel/posix-cpu-timers.c:

        unsigned long psecs = cputime_to_secs(ptime);
        ...
        if (psecs >= sig->rlim[RLIMIT_CPU].rlim_max) {
                ...
                __group_send_sig_info(SIGKILL, SEND_SIG_PRIV, tsk);

Signed-off-by: Kees Cook <kees.cook@canonical.com>
Acked-by: WANG Cong <xiyou.wangcong@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>