Fix LUClusterRepairDiskSizes and rpc result usage

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>
(cherry picked from commit 043beb38f4e10b75d0820c361c668c441c7a6980)