fs/dlm: use list_add_tail() instead of open-coding list insertion

fs/dlm: use list_add_tail() instead of open-coding list insertion

Replace the manual list pointer manipulation in add_ordered_member()
with the standard list_add_tail() helper. The original code explicitly
updated ->prev and ->next pointers to insert @newlist before @tmp,
which is exactly what list_add_tail(newlist, tmp) provides.

Using the list macro improves readability, removes a source of
potential pointer bugs, and satisfies the existing FIXME requesting
conversion to the list helpers. No functional change in the ordering
logic for DLM members.

Signed-off-by: Shaurya Rane <ssrane_b23@ee.vjti.ac.in>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>