This fixes the integer underflow in task_hot() noticed by Kenneth W Chen
and makes use of p->last_ran to separate load-balancing timestamps (used
by task_hot()) from interactivity timestamps.  (which two interfered)

compiled, booted on x86 SMP.

Confirmed by Kenneth Chen <kenneth.w.chen@intel.com> to fix the db
transaction processing workload that showed the balancing problem.
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>