Port the mps panic-safe shutdown_final handling to mpr

r330951 by smh fixed the mps driver to avoid deadlocks when panicing.
The same code is needed for mpr, so port it here, along with the fix
which allows the CCBs scheduled to complete avoiding at least a scary
message and likely other unintended consequences.

Sponsored by: Netflix
Differential Review: https://reviews.freebsd.org/D16663