fs: dlm: check on plock ops when exit dlm

To be sure we don't have any issues that there are leftover plock ops in
either send_list or recv_list we simple check if either one of the list
are empty when we exit the dlm subsystem.

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