net: hso: fix null-ptr-deref during tty device unregistration

In the Linux kernel, the following vulnerability has been resolved:

net: hso: fix null-ptr-deref during tty device unregistration

Multiple ttys try to claim the same the minor number causing a double

unregistration of the same device. The first unregistration succeeds

but the next one results in a null-ptr-deref.

The get_free_serial_index() function returns an available minor number

but doesn't assign it immediately. The assignment is done by the caller

later. But before this assignment, calls to get_free_serial_index()

would return the same minor number.

Fix this by modifying get_free_serial_index to assign the minor number

immediately after one is found to be and rename it to obtain_minor()

to better reflect what it does. Similary, rename set_serial_by_index()

to release_minor() and modify it to free up the minor number of the

given hso_serial. Every obtain_minor() should have corresponding

release_minor() call.