NFSD: Report zero space limit for write delegations

Replace the -1 (no limit) with a zero (no reserved space).

This prevents certain non-determinant client behavior, such as
silly-renaming a file when the only open reference is a write
delegation. Such a rename can leave unexpected .nfs files in a
directory that is otherwise supposed to be empty.

Note that other server implementations that support write delegation
also set this field to zero.

Suggested-by: Dai Ngo <dai.ngo@oracle.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>