tty: audit: do not use N_TTY_BUF_SIZE

tty: audit: do not use N_TTY_BUF_SIZE

N_TTY_BUF_SIZE -- as the name suggests -- is the N_TTY's buffer size.
There is no reason to couple that to audit's buffer size, so define an
own TTY_AUDIT_BUF_SIZE macro (with the same size).

N_TTY_BUF_SIZE is private and will be moved to n_tty.c later.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-3-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>