gfs2: a minor finish_xmote cleanup

As a minor clean-up to commit 1fc05c8d ("gfs2: cancel timed-out
glock requests"), when a demote request is in progress in
finish_xmote(), there is no point in waking up the glock holder at the
head of the queue because the reply from dlm cannot be on behalf of that
glock holder.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>