tty: audit: Take siglock directly

lock_task_sighand() is for situations where the struct task_struct*
may disappear while trying to deref the sighand; this never applies
to 'current'.

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