[PATCH] Fix idle time field in /proc/uptime

Git commit 79741dd3

 changes idle cputime accounting, but unfortunately
the /proc/uptime file hasn't caught up.  Here the idle time calculation
from /proc/stat is copied over.

Signed-off-by: Michael Abbott <michael.abbott@diamond.ac.uk>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>