powerpc/mm: Set fault flags earlier

Move out the code that sets FAULT_FLAG_WRITE so the block that check
access permissions can be extracted. While at it also set
FAULT_FLAG_INSTRUCTION which will be used for protection keys.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>