powerpc/smp: Optimize update_mask_by_l2

All threads of a SMT4 core can either be part of this CPU's l2-cache
mask or not related to this CPU l2-cache mask. Use this relation to
reduce the number of iterations needed to find all the CPUs that share
the same l2-cache.

Use a temporary mask to iterate through the CPUs that may share l2_cache
mask. Also instead of setting one CPU at a time into cpu_l2_cache_mask,
copy the SMT4/sub mask at one shot.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200921095653.9701-10-srikar@linux.vnet.ibm.com