generic: use correct size value in generic/273

generic: use correct size value in generic/273

generic/273 factors the "space available" output from df into the
calculation for the size of the origin data set. Recent commit

  bfdd1e72b358 xfstests: added -P option to $DF_PROG

... converted the use of 'df' to $DF_PROG. This implicitly adds the
-T parameter to add the fs type column, shifts the available space
column over by one and unintentionally causes 273 to look at "used
space" and create too small of a data set for a useful test.
Realign to the available space value.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>