mmu-hash64: Factor SLB N bit into permissions bits

BEHAVIOUR CHANGE

Currently, for 64-bit hash mmu, the execute protection bit placed into the
qemu tlb is based only on the N (No execute) bit from the PTE.  However,
No Execute can also be set at the segment level.  We do check this on
execute faults, but this still means we could incorrectly allow execution
of code from a No Execute segment, if a prior read or write fault caused
the page to be loaded into the qemu tlb with PROT_EXEC set.

To correct this, we (re-)check the segment level no execute permission when
generating the protection bits for the qemu tlb.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
