Fix mps deadlock when handling panic

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.

Reviewed by: slm
MFC after: 1 week
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D12776