NFSD: Shrink size of struct nfsd4_copy

struct nfsd4_copy is part of struct nfsd4_op, which resides in an
8-element array.

sizeof(struct nfsd4_op):
Before: /* size: 1696, cachelines: 27, members: 5 */
After:  /* size: 672, cachelines: 11, members: 5 */

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