xfstests: 298: Test that FS sends discard requests only on free blocks

xfstests: 298: Test that FS sends discard requests only on free blocks

V1->V2: Change way of testing suggested by Dave Chinner
1. Create image file with FS on it
2. Call fstrim to discard free blocks
3. Check that every punched hole in the image file is in the area
   that is marked as free

Signed-off-by: Tomas Racek <tracek@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>