ZTS: Introduce targeted corruption in file blocks

filetest_001_pos verifies that various checksum algorithms detect
corruption by overwriting the underlying vdev on which a file resides.
It is possible for the overwrite to miss the blocks of a file, causing a
spurious failure. This change introduces a function to corrupt the
individual blocks of a file as determined by zdb.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #9288