fs: dlm: let handle callback data as void

This patch changes the dlm_lowcomms_new_msg() function pointer private data
from "struct mhandle *" to "void *" to provide different structures than
just "struct mhandle".

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>