[DLM] down conversion clearing flags

The down-conversion optimization was resulting in the lkb flags being
cleared because the stub message reply had no flags value set.  Copy the
current flags into the stub message so they'll be copied back into the lkb
as part of processing the fake reply.  Also add an assertion to catch this
error more directly if it exists elsewhere.

Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>