dlm: use proper C for dlm/requestqueue stuff (and fix alignment bug)

a) don't cast the pointer to dlm_header *, we use it as dlm_message *
   anyway.
b) we copy the message into a queue element, then pass the pointer to
   copy to dlm_receive_message_saved(); declare it properly to make sure
   that we have the right alignment.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David Teigland <teigland@redhat.com>