exec: Move the comment from above de_thread to above unshare_sighand

The comment describes work that now happens in unshare_sighand so
move the comment where it makes sense.

Link: https://lkml.kernel.org/r/87mu6i6zcs.fsf_-_@x220.int.ebiederm.org


Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>