powerpc/kprobes: Don't save/restore DAR/DSISR to/from pt_regs for optprobes

We don't save/restore these across a trap, or with KPROBES_ON_FTRACE.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>