SUNRPC: Shrink size of struct rpc_task

Move the field 'tk_rpc_status' so that we eliminate a 4 byte hole in the
structure.
For x86_64, this shrinks the size of the struct by 8 bytes.

'pahole' output before the change:
        /* size: 232, cachelines: 4, members: 27 */
        /* sum members: 222, holes: 1, sum holes: 4 */
        /* sum bitfield members: 8 bits (1 bytes) */
        /* padding: 5 */
        /* last cacheline: 40 bytes */

'pahole' output after the change:
        /* size: 224, cachelines: 4, members: 27 */
        /* padding: 1 */
        /* last cacheline: 32 bytes */

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>