NFSD: provide encode routine for OP_OPENATTR

Although this operation is unsupported by our implementation
we still need to provide an encode routine for it to
merely encode its (error) status back in the compound reply.

Thanks for Bill Baker at sun.com for testing with the Sun
OpenSolaris' client, finding, and reporting this bug at
Connectathon 2009.

This bug was introduced in 2.6.27

Signed-off-by: Benny Halevy <bhalevy@panasas.com>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>