CVE-2021-47358

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

staging: greybus: uart: fix tty use after free

User space can hold a tty open indefinitely and tty drivers must not
release the underlying structures until the last user is gone.

Switch to using the tty-port reference counter to manage the life time
of the greybus tty state to avoid use after free after a disconnect.