usb-serial: fix mos_parport refcount imbalance on error path

Hi all,

We might find a security issue in function 'write_parport_reg_nonbloc'(for source codes, please refer to
https://github.com/torvalds/linux/blob/master/drivers/usb/serial/mos7720.c),
which is logically an unbalanced reference count vulnerability.

Let us check ref_count 'mos_parport->ref_count' of kernel object 'mos_parport' in the function. In 'Line 369:
kref_get(&mos_parport->ref_count)' and 'Line 370:
urbtrack->mos_parport = mos_parport', the ref_count has been increased because of a new variable 'urbtrack->mos_parport' pointing to 'mos_parport', that make sense to balance ref_count. However, at 'Line 373 and Line 379: kfree(urbtrack)', variable 'urbtrack' has been freed before a return, which will make 'urbtrack->mos_parport' freed, too. But, a decreasing of 'mos_parport->ref_count' is missing in the current kernel version, which will bring an unbalanced reference count.

We have wrote a patch showed in Lin's email. The idea is simple. We added a calling of 'kref_put(&mos_parport->ref_count, destroy_mos_parport)' below Line 373 and Line 379. Currently, we are not sure if this is really a vulnerability. If you are familiar with codes, please help to verify it.