powerpc/64: hard disable irqs in panic_smp_self_stop

Similarly to commit 855bfe0d ("powerpc: hard disable irqs in
smp_send_stop loop"), irqs should be hard disabled by
panic_smp_self_stop.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>