nfsd: trivial GET_DIR_DELEGATION support

This adds basic infrastructure for handing GET_DIR_DELEGATION calls from
clients, including the decoders and encoders. For now, it always just
returns NFS4_OK + GDD4_UNAVAIL.

Eventually clients may start sending this operation, and it's better if
we can return GDD4_UNAVAIL instead of having to abort the whole compound.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>