nfs: verify open flags before allowing an atomic open

Currently, you can open a NFSv4 file with O_APPEND|O_DIRECT, but cannot
fcntl(F_SETFL,...) with those flags. This flag combination is explicitly
forbidden on NFSv3 opens, and it seems like it should also be on NFSv4.

Reported-by: Chao Ye <cye@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>