NFSD: Add nfsd4_encode_open_read_delegation4()

Refactor nfsd4_encode_open() so the open_read_delegation4 type is
encoded in a separate function. This makes it more straightforward
to later add support for returning an nfsace4 in OPEN responses that
offer a delegation.

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