xfsqa: fix size specification for scratch mkfs

xfsqa: fix size specification for scratch mkfs

When making a specifically sized scratch filesystem, we need to
calculate the size rather than assigning the caclculation expression
to a variable and passing that into the _scratch_mkfs_sized
function. Currently sized filesystems are failing to be made and the
code is falling back to full sized filesystems so we are not testing
enospc issues correctly.

Signed-off-by: Dave Chinner <david@fromorbit.com>