Fix LUClusterRepairDiskSizes and rpc result usage

This LU was introduced before the RPC result conversion from .data to
.payload, and it has managed to keep the old-style usage (how? it's
the only LU that does so). Fix by changing to payload, and add some
extra logging for easier diagnose.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Stephen Shirley <diamond@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>