fs: dlm: unregister memory at the very last

The dlm modules midcomms, debugfs, lockspace, uses kmem caches. We
ensure that the kmem caches getting deallocated after those modules
exited.

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