NFS: Fix a race in __nfs_list_for_each_server()

The struct nfs_server gets put on the cl_superblocks list before
the server->super field has been initialised, in which case the
call to nfs_sb_active() will Oops. Add a check to ensure that
we skip such a list entry.

Fixes: 3c9e502b ("NFS: Add a helper nfs_client_for_each_server()")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/super.c

@@ -185,7 +185,7 @@ static int __nfs_list_for_each_server(struct list_head *head,
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(server, head, client_link) {
-		if (!nfs_sb_active(server->super))
+		if (!(server->super && nfs_sb_active(server->super)))
 			continue;
 		rcu_read_unlock();
 		if (last)