Yama: allow access for the current ptrace parent

Under ptrace_scope=1, it's possible to have a tracee that is already
ptrace-attached, but is no longer a direct descendant.  For instance, a
forking daemon will be re-parented to init, losing its ancestry to the
tracer that launched it.

The tracer can continue using ptrace in that state, but it will be
denied other accesses that check PTRACE_MODE_ATTACH, like process_vm_rw
and various procfs files.  There's no reason to prevent such access for
a tracer that already has ptrace control anyway.

This patch adds a case to ptracer_exception_found to allow access for
any task in the same thread group as the current ptrace parent.

Signed-off-by: Josh Stone <jistone@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: James Morris <james.l.morris@oracle.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: James Morris <james.l.morris@oracle.com>