Fixed Regression in NFS Direct I/O path

A typo, introduced by commit f11ac8db, in the nfs_direct_write()
routine causes writes with O_DIRECT set to fail with a ENOMEM error.

Found-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>