NFSD: Shrink size of struct nfsd4_copy_notify

struct nfsd4_copy_notify is part of struct nfsd4_op, which resides
in an 8-element array.

sizeof(struct nfsd4_op):
Before: /* size: 2208, cachelines: 35, members: 5 */
After:  /* size: 1696, cachelines: 27, members: 5 */

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>