move thread switch tracing from mi_switch to sched_switch

move thread switch tracing from mi_switch to sched_switch

This is done so that the thread state changes during the switch
are not confused with the thread state changes reported when the thread
spins on a lock.

Here is an example, three consecutive entries for the same thread (from top to
bottom):

  KTRGRAPH group:"thread", id:"zio_write_intr_3 tid 100260", state:"sleep", attributes: prio:84, wmesg:"-", lockname:"(null)"
  KTRGRAPH group:"thread", id:"zio_write_intr_3 tid 100260", state:"spinning", attributes: lockname:"sched lock 1"
  KTRGRAPH group:"thread", id:"zio_write_intr_3 tid 100260", state:"running", attributes: none

The above trace could leave an impression that the final state of
the thread was "running".
After this change the sleep state will be reported after the "spinning"
and "running" states reported for the sched lock.

Reviewed by:	jhb, markj
MFC after:	1 week
Sponsored by:	Panzura
Differential Revision: https://reviews.freebsd.org/D9961