Reorder structure writeback_control to remove 8 bytes of padding on 64
bit builds, this shrinks its size from 48 to 40 bytes.

This structure is always on the stack and uses C99 named initialisation,
so should be safe and have a small impact on stack usage.
Signed-off-by: NRichard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>