NFSv4.1: Fix a refcounting issue in the pNFS device id cache

When we add something to the global device id cache, we need to bump the
reference count, so that the cache itself holds a reference.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/pnfs_dev.c b/fs/nfs/pnfs_dev.c
index 5944d4b..f0f8e1e 100644
--- a/fs/nfs/pnfs_dev.c
+++ b/fs/nfs/pnfs_dev.c
@@ -209,6 +209,7 @@
 
 	hlist_add_head_rcu(&new->node, &nfs4_deviceid_cache[hash]);
 	spin_unlock(&nfs4_deviceid_lock);
+	atomic_inc(&new->ref);
 
 	return new;
 }