NFSD: Add nfsd4_encode_open_delegation4()

To better align our implementation with the XDR specification,
refactor the part of nfsd4_encode_open() that encodes delegation
metadata.

As part of that refactor, remove an unnecessary BUG() call site and
a comment that appears to be stale.

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