NFSv4 do not send an empty SETATTR compound

Commit 536e43d12b9517bbbf6114cd1a12be27857a4d7a ATTR_OPEN check can result in
an ia_valid with only ATTR_FILE set, and no NFS_VALID_ATTRS attributes to
request from the server.

Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>