fs: dlm: use packet in dlm_mhandle

[ Upstream commit 5b787667 ]

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>
Stable-dep-of: 724b6bab ("fs: dlm: fix use after free in midcomms commit")
Signed-off-by: Sasha Levin <sashal@kernel.org>