Refine packed annotations in stat.h

Refine packed annotations in stat.h

Instead of declaring the whole structure packed, only declare non-aligned
members packed. This patch is an alternative way to fix the following gcc 9
compiler warnings:

eta.c: In function 'calc_thread_status':
eta.c:510:7: error: taking address of packed member of 'struct jobs_eta' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  510 |     je->rate);
      |     ~~^~~~~~
eta.c:522:66: error: taking address of packed member of 'struct jobs_eta' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  522 |  calc_rate(unified_rw_rep, disp_time, io_bytes, disp_io_bytes, je->rate);
      |                                                                ~~^~~~~~
eta.c:523:64: error: taking address of packed member of 'struct jobs_eta' may result in an unaligned pointer value [-Werror=address-of-packed-member]
  523 |  calc_iops(unified_rw_rep, disp_time, io_iops, disp_io_iops, je->iops);
      |

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>