ZTS: Add reservation_008_pos exception

ZTS: Add reservation_008_pos exception

The reservation_008_pos test case has been observed to fail in
a non-dangerous way in approximately 5% of automated test runs.
Add the test case to the list of possible expected failures
until the test case can be made perfectly reliable.

Reviewed by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #7741
Closes #7742