ZTS: fallocate tests fail with hard coded values

Currently, these two tests pass on disks with 512 byte sectors. In
environments where the backing store is different, the number of
blocks allocated to write the same file may differ. This change
modifies the reported size check to detect an expected change in the
reported number of blocks without specifying a particular number.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #13931