powerpc/smp: Adjust nr_cpu_ids to cover all threads of a core

If nr_cpu_ids is too low to include at least all the threads of a single
core adjust nr_cpu_ids upwards. This avoids triggering odd bugs in code
that assumes all threads of a core are available.

Cc: stable@vger.kernel.org
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231229120107.2281153-1-mpe@ellerman.id.au