During shutdown mps waits for its SSU requests to complete however when
performing a reboot after handling a panic the scheduler is stopped so
getmicrotime which is used can be non-functional.
Switch to using the same method as shutdown_panic to ensure we actually
complete.
In addition reduce the timeout when RB_NOSYNC is set in howto as we expect
this to fail.
This is r330951 adapted for mpr(4)