powerpc/mm: Move the DSISR_PROTFAULT sanity check

This has a page of comment explaining what's going on right in
the middle of do_page_fault() which makes things a bit hard to
follow. Move it to a helper instead. Also do the test earlier
as there's no point waiting until after we found the VMA.

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