ns3-dce: Add return-statement to register_shrinker

Otherwise, certain compiler will complain that the function returns void in a
function returning a non-void value.

Fixes: 1e9aa348d8b3 (adapt new API around v3.11-rc7)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
ns3-dce: Correctly initialize lru-list

Since v3.12, s_dentry_lru and s_inode_lru are lru-lists.
Patch 1e9aa348d8b (adapt new API around v3.11-rc7) changes the handling
so the attempt to init the spinlock is no more done. However, the list
must also be initialized with list_lru_init.

Fixes: 1e9aa348d8b3 (adapt new API around v3.11-rc7)
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>