powerpc: Fix pte_update for CONFIG_PTE_64BIT and !PTE_ATOMIC_UPDATES

Because the pte is now 64-bits the compiler was optimizing the update
to always clear the upper 32-bits of the pte.  We need to ensure the
clr mask is treated as an unsigned long long to get the proper behavior.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>