ZTS: Retire "tmpfile_reason" exception
All supported Linux kernels, 4.18 and newer, provide O_TMPFILE.
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16553