ppc: Fix catching some segfaults in user mode

The usermode "translate" code generates an error code value that
has the "is_write" bit set, which causes our switch/case to miss
and display "Invalid segfault errno" and a spurrious second state
dump. Fix it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
