powerpc/mm: Use 32bit array for slb cache

With larger vsid we need to track more bits of ESID in slb cache
for slb invalidate.

Reviewed-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>