CVE-2026-23472: serial: core: fix infinite loop in handle_tx() for PORT_UNKNOWN

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

serial: core: fix infinite loop in handletx() for PORTUNKNOWN

uartwriteroom() and uartwrite() behave inconsistently when xmitbuf is NULL (which happens for PORTUNKNOWN ports that were never properly initialized):

- uartwriteroom() returns kfifoavail() which can be > 0 - uartwrite() checks xmitbuf and returns 0 if NULL

This inconsistency causes an infinite loop in drivers that rely on ttywriteroom() to determine if they can write:

while (ttywriteroom(tty) > 0) { written = tty->ops->write(...); // written is always 0, loop never exits }

For example, caifserial's handletx() enters an infinite loop when used with PORTUNKNOWN serial ports, causing system hangs.

Fix by making uartwriteroom() also check xmitbuf and return 0 if it's NULL, consistent with uartwrite().

Reproducer: https://gist.github.com/mrpre/d9a694cc0e19828ee3bc3b37983fde13