NFSD: Finish converting the NFSv3 GETACL result encoder

For some reason, the NFSv2 GETACL result encoder was fully converted
to use the new nfs_stream_encode_acl(), but the NFSv3 equivalent was
not similarly converted.

Fixes: 20798dfe

 ("NFSD: Update the NFSv3 GETACL result encoder to use struct xdr_stream")
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>