sched/cputime: sig->prev_stime underflow

[Dave Hansen <dave@sr71.net>]

With the 3.9-rcs (and probably much earlier) I'm seeing some weird top
output where the cpu time "spent" is millions of hours:

445 root      20   0     0    0    0 S    0  0.0  5124095h kworker/45:1
404 root      20   0     0    0    0 S    0  0.0  5124095h kworker/4:1

I see it mostly with kernel threads, but it doesn't seem to happen on my
distro kernel (3.5 era).  The suspect code is in thread_group_times():

	sig->prev_stime = max(sig->prev_stime, rtime - sig->prev_utime);

In my case, I caught it with rtime=34 and sig->prev_utime=35.  This code
_looks_ to be pretty mature, coming in at commit 0cf55e1e in 2009.  The
system I'm running on _does_ have some non-sync'd TSCs, but they are at
least being detected, so I expect the fallout to be minimal:

	tsc: Marking TSC unstable due to check_tsc_sync_source failed

config:

	http://sr71.net/~dave/linux/config-bigbox-04042013.txt

The dumb fix here would seem to be to just check "rtime <
sig->prev_utime".  Any thoughts?