btrfs: reorder members in btrfs_delayed_root for better packing

There are two unnecessary 4B holes in btrfs_delayed_root;

struct btrfs_delayed_root {
        spinlock_t                 lock;                 /*     0     4 */

        /* XXX 4 bytes hole, try to pack */

        struct list_head           node_list;            /*     8    16 */
        struct list_head           prepare_list;         /*    24    16 */
        atomic_t                   items;                /*    40     4 */
        atomic_t                   items_seq;            /*    44     4 */
        int                        nodes;                /*    48     4 */

        /* XXX 4 bytes hole, try to pack */

        wait_queue_head_t          wait;                 /*    56    24 */

        /* size: 80, cachelines: 2, members: 7 */
        /* sum members: 72, holes: 2, sum holes: 8 */
        /* last cacheline: 16 bytes */
};

Reordering 'nodes' after 'lock' reduces size by 8B, to 72 on release
config.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>