ZTS: update upgrade_readonly_pool.ksh

Modify the test case to use the zfs mount command instead
of directly calling the mount command, create a dedicated dataset,
and use the default mount point. These changes are intended to
preserve the intent of the original test case and resolve some
spurious mount failures which have been observed by the CI.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17785