powerpc/smp: Remove unnecessary variable

Commit 3ab33d6d ("powerpc/smp: Optimize update_mask_by_l2")
introduced submask_fn in update_mask_by_l2 to track the right submask.
However commit f6606cfd ("powerpc/smp: Dont assume l2-cache to be
superset of sibling") introduced sibling_mask in update_mask_by_l2 to
track the same submask. Remove sibling_mask in favour of submask_fn.

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/20201019042716.106234-2-srikar@linux.vnet.ibm.com