Change the hugepage directory format so that we can have leaf ptes directly
at page directory avoiding the allocation of hugepage directory.

With the new table format we have 3 cases for pgds and pmds:
(1) invalid (all zeroes)
(2) pointer to next table, as normal; bottom 6 bits == 0
(4) hugepd pointer, bottom two bits == 00, next 4 bits indicate size of table

Instead of storing shift value in hugepd pointer we use mmu_psize_def index
so that we can fit all the supported hugepage size in 4 bits
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Acked-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>