powerpc/smp: Optimize start_secondary

In start_secondary, even if shared_cache was already set, system does a
redundant match for cpumask. This redundant check can be removed by
checking if shared_cache is already set.

While here, localize the sibling_mask variable to within the if
condition.

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200810071834.92514-7-srikar@linux.vnet.ibm.com