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>