powerpc: Add write barrier before enabling DTL flags

Currently, we don't enforce any ordering for updates to the lppaca
when enabling dtl logging, so we may end up enabling logging before the
index fields have been established.

This change adds a smp_wmb() before doing the actual enable.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>