When both stolen and blocked are rounded down, it is possible for the

When both stolen and blocked are rounded down, it is possible for the
final increment of the cpu local processed_system_time to move the cpu
local system time ahead a bit further than expected - but still proper
wrt. wall clock time.

This gives problems on a next timer interrupt, when stolen and blocked
both get rounded up and end up incrementing the per cpu
processed_system_time too far.

These simple checks make sure that the cpu local processed_system_time
never gets advanced too far.  Not advancing the variable now should be
fine, since we'll increment it at the next timer tick...

This patch has made the "time went backwards" error messages go away
completely.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Keir Fraser <keir@xensource.com>