powerpc: Optimise smp_rmb

After commit 598056d5 ("[POWERPC] Fix
rmb to order cacheable vs. noncacheable"), rmb() becomes a sync
instruction, which is needed to order cacheable vs noncacheable loads.
However smp_rmb() is #defined to rmb(), and smp_rmb() can be an
lwsync.

This restores smp_rmb() performance by using lwsync there and updates
the comments.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>