NFSD: Reorder the fields in struct nfsd4_op

Pack the fields to reduce the size of struct nfsd4_op, which is used
an array in struct nfsd4_compoundargs.

sizeof(struct nfsd4_op):
Before: /* size: 672, cachelines: 11, members: 5 */
After:  /* size: 640, cachelines: 10, members: 5 */

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