tty: Move sysctl setup into "core" tty logic

In preparation for adding another sysctl to the tty subsystem, move the
tty setup code into the "core" tty code, which contains tty_init() itself.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221022182949.2684794-1-keescook@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>