stop_machine() guarantees that all online CPUs are spinning
non-preemptible in a known code location before a subset of them may
safely run a stop-context function. This service is typically useful
for live patching the kernel code, or changing global memory mappings,
so that no activity could run in parallel until the system has
returned to a stable state after all stop-context operations have
completed.

When interrupt pipelining is enabled, we have to provide the same
guarantee by restoring hard interrupt disabling where virtualizing the
interrupt disable flag would defeat it.