Fix ztest_vdev_add_remove() test case

Commit 2ffd89fc allowed two new errors to be reported by zil_reset()
in order to provide a descriptive error message regarding why a log
device could not be removed. However, the new return values were
not handled in the ztest_vdev_add_remove() test case resulting in
ztest failures during automated testing.

Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7630