CVE-2024-26998: serial: core: Clearing the circular buffer before NULLifying it

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

serial: core: Clearing the circular buffer before NULLifying it

The circular buffer is NULLified in uartttyportshutdown() under the spin lock. However, the PM or other timer based callbacks may still trigger after this event without knowning that buffer pointer is not valid. Since the serial code is a bit inconsistent in checking the buffer state (some rely on the head-tail positions, some on the buffer pointer), it's better to have both aligned, i.e. buffer pointer to be NULL and head-tail possitions to be the same, meaning it's empty. This will prevent asynchronous calls to dereference NULL pointer as reported recently in 8250 case:

BUG: kernel NULL pointer dereference, address: 00000cf5 Workqueue: pm pmruntimework EIP: serial8250txchars (drivers/tty/serial/8250/8250port.c:1809) ... ? serial8250txchars (drivers/tty/serial/8250/8250port.c:1809) starttx (drivers/tty/serial/8250/8250port.c:1551) serial8250starttx (drivers/tty/serial/8250/8250port.c:1654) serialportruntimesuspend (include/linux/serialcore.h:667 drivers/tty/serial/serialport.c:63) rpmcallback (drivers/base/power/runtime.c:393) ? serialportremove (drivers/tty/serial/serialport.c:50) rpmsuspend (drivers/base/power/runtime.c:447)

The proposed change will prevent ->starttx() to be called during suspend on shut down port.