dlm: fix lvb copy for user locks

For a userland lock request, the previous and current
lock modes are used to decide when the lvb should be
copied back to the user.  The wrong previous value was
used, so that it always matched the current value.
This caused the lvb to be copied back to the user in
the wrong cases.

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