Use boot based time for uptime in /proc

Commit 411187fb caused uptime not to increase
during suspend.  This may cause confusion so I restore the old behaviour by
using the boot based time instead of monotonic for uptime.

Signed-off-by: Tomas Janousek <tjanouse@redhat.com>
Acked-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>