powerpc/64s: Fix hash__change_memory_range preemption warning

stop_machine_cpuslocked takes a mutex so it must be called in a
preemptible context, so it can't simply be fixed by disabling
preemption.

This is not a bug, because CPU hotplug is locked, so this processor will
call in to the stop machine function. So raw_smp_processor_id() could be
used. This leaves a small chance that this thread will be migrated to
another CPU, so the master work would be done by a CPU from a different
context. Better for test coverage to make that a common case by just
having the first CPU to call in become the master.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221013151647.1857994-2-npiggin@gmail.com