fs: dlm: use packet in dlm_mhandle

To allow more than just dereferencing the inner header we directly point
to the inner dlm packet which allows us to dereference the header, rcom
or message structure.

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