ztest: Fix scrub check in ztest_raidz_expand_check()

The scrub code may return EBUSY under several possible scenarios
causing ztest to incorrectly ASSERT when verifying the result of
a raidz expansion. Update the test case to allow EBUSY since it
does not indicate pool damage.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16627