tty: add the option to have a tty reject a new ldisc

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: tty: add the option to have a tty reject a new ldisc ... and use it to limit the virtual terminals to just N_TTY. They are kind of special, and in particular, the "con_write()" routine violates the "writes cannot sleep" rule that some ldiscs rely on. This avoids the BUG: sleeping function called from invalid context at kernel/printk/printk.c:2659 when N_GSM has been attached to a virtual console, and gsmld_write() calls con_write() while holding a spinlock, and con_write() then tries to get the console lock.

References

• https://git.kernel.org/stable/c/3c6332f3bb1578b5b10ac2561247b1d6272ae937
• https://git.kernel.org/stable/c/287b569a5b914903ba7c438a3c0dbc3410ebb409
• https://git.kernel.org/stable/c/5920ac19964f9e20181f63b410d9200ddbf8dc86
• https://git.kernel.org/stable/c/6bd23e0c2bb6c65d4f5754d1456bc9a4427fc59b