dlm: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the corresponding
structure. Notice that `struct dlm_message` is a flexible
structure, this is a structure that contains a flexible-array
member.

Fix the following warning:

fs/dlm/dlm_internal.h:609:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David Teigland <teigland@redhat.com>