sched: rework and rename vcpu_force_reschedule()

sched: rework and rename vcpu_force_reschedule()

vcpu_force_reschedule() is only used for modifying the periodic timer
of a vcpu. Forcing a vcpu to give up the physical cpu for that purpose
is kind of brutal.

So instead of doing the reschedule dance just operate on the timer
directly. By protecting periodic timer modifications against concurrent
timer activation via a per-vcpu lock it is even no longer required to
bother the target vcpu at all for updating its timer.

Even with the additional lock there is not more serialization involved
compared to the current solution, as today's de-scheduling the vcpu is
requiring to take the schedule lock, which has a much higher contention
probability than the new lock.

Rename the function to vcpu_set_periodic_timer() as this now reflects
the functionality.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Dario Faggioli <dfaggioli@suse.com>