powerpc: Hugetlb pgtable cache access cleanup

Andrew Morton suggested that using a macro that makes an array
reference look like a function call makes it harder to understand the
code.

This therefore removes the huge_pgtable_cache(psize) macro and
replaces its uses with pgtable_cache[HUGE_PGTABLE_INDEX(psize)].

Signed-off-by: Jon Tollefson <kniht@linux.vnet.ibm.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>