ZTS: Add dirty dnode stress test

Add a test for the dirty dnode SEEK_HOLE/SEEK_DATA bug described in
https://github.com/openzfs/zfs/issues/15526

The bug was fixed in https://github.com/openzfs/zfs/pull/15571 and
was backported to 2.2.2 and 2.1.14. This test case is just to
make sure it does not come back.

seekflood.c originally written by Rob Norris.

Reviewed-by: Graham Perrin <grahamperrin@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #15608