NFS: Define nfs_direct_write_schedule_work() when v3 and v4 are disabled

v2 doesn't have commits, so this function can be a no-op.
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/direct.c

@@ -601,6 +601,9 @@ static void nfs_direct_write_complete(struct nfs_direct_req *dreq, struct inode
 }
 
 #else
+static void nfs_direct_write_schedule_work(struct work_struct *work)
+{
+}
 
 static void nfs_direct_write_complete(struct nfs_direct_req *dreq, struct inode *inode)
 {