Reduce the scope of reset_mutext to protect the msleep in the watch dog
thread as well as the MPI3MR_FLAGS_SHUTDOWN bit. Use it to protect the
wakeup in mpi3mr_detach so this thread can exit sooner when we're trying
to do an orderly shutdown.
It's an open question if this should protect sc->unrecoverable, and if
we should wakeup the watchdog thread when we set it.