Clear the internal state ourselves before raising error.

If we need to raise an error, make sure the internal state is clean, because
a successful driver.Get() may have its internal state changed (eg. counting,
or mounts), while callers will only do that after a succussful Mount().

Signed-off-by: shuai-z <zs.broccoli@gmail.com>
Merge pull request #8850 from shuai-z/clean-mount

Clear the internal state before raising error.