powerpc/cell: Fix iommu exception reporting

Currently, we will report a page fault as a segment fault, and report
a segment fault as both a page and segment fault.

Fix the SPF_P definition to be correct according to the iommu docs, and
mask before comparing.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>