dtrace: vtimestamp implementation

dtrace: vtimestamp implementation

This commit adds DTrace vtimestamp support.  It keeps track of how much
time a task has spent actually processing on a CPU.  The time is set to
zero at task creation, and is updated whenever the task leaves a CPU
(gets scheduled off), and when the dtrace_probe() function is entered,
to enusre that the most recent value of consumed time is reported.

Some code got moved around for consistency of the implementation.

Orabug: 17741477

Reviewed-by: Dan Duval <dan.duval@oracle.com>
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Conflicts:
	include/linux/ktime.h
	kernel/sched/core.c