xfs/074: specify filesystem size in terms of size not block count

xfs/074: specify filesystem size in terms of size not block count

When testing 512 block size xfs, xfs/074 fails as

   QA output created by 074
  +fallocate: No space left on device
   Silence is golden

That's because 40051712*512=20G < 30G.

And quote from Dave:

  That was sized to give AGs of a specific size, which originally
  contributed to the problem being exposed. You should change the block
  count specification to a size specification so the filesysetm being
  made on 4k block size filesystems remains unchanged.

  size    = 40051712b
          = 40051712 * 4096
          = 164,051,812,352
          = 156452m

So set the filesystem size to 156452m explicitly.

Suggested-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>