powerpc/mm/coproc: Handle bad address on coproc slb fault

VSID 0 is bad address. Don't create slb entries on coproc fault for
bad address

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Reviewed-by: Balbir Singh <bsingharora@gmail.com>
Reviewed-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>