PPC: Fix TLB invalidation bug within the PPC interrupt handler.

Commit 41557447d30eeb944e42069513df13585f5e6c7f also introduced a subtle TLB
flush bug. By applying a mask to the interrupt MSR which cleared the IR/DR
bits at the start of the interrupt handler, the logic towards the end of the
handler to force a TLB flush if either one of these bits were set would never
be triggered.

This patch simply changes the IR/DR bit check in the TLB flush logic to use
the original MSR value (albeit with some interrupt-specific bits cleared) so
that the IR/DR bits are preserved at the point where the check takes place.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Andreas Färber <afaerber@suse.de>
