tty: audit: Remove tty_audit_buf reference counting

When tty_audit_exit() is called from do_exit(), the process is
single-threaded. Since the tty_audit_buf is only shared by threads
of a process, no other thread can be concurrently accessing the
tty_audit_buf during or after tty_audit_exit().

Thus, no other thread can be holding an extra tty_audit_buf reference
which would prevent tty_audit_exit() from freeing the tty_audit_buf.
As that is the only purpose of the ref counting, remove the reference
counting and free the tty_audit_buf directly.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>