UPSTREAM: arm/ptrace: run seccomp after ptrace

Close the hole where ptrace can change a syscall out from under seccomp.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: linux-arm-kernel@lists.infradead.org
(cherry picked from commit 0f3912fd934cdfd03d93f2dc6f064099795bf638)

Bug: 119769499
Change-Id: Id82e4137207db42a8af31b2745581c53eaaf1f89
Signed-off-by: Greg Hackmann <ghackmann@google.com>