ZTS: Fix checkpoint_ro_rewind.ksh

Related to commit 90b77a036. Retry the zpool export if the pool is
"busy" indicating there is a process accessing the mount point. This
can happen after an import and allowing it to be retried will avoid
spurious test failures.

Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13092