seccomp, ptrace: switch get_metadata types to arch independent

Commit 26500475 ("ptrace, seccomp: add support for retrieving seccomp
metadata") introduced `struct seccomp_metadata`, which contained unsigned
longs that should be arch independent. The type of the flags member was
chosen to match the corresponding argument to seccomp(), and so we need
something at least as big as unsigned long. My understanding is that __u64
should fit the bill, so let's switch both types to that.

While this is userspace facing, it was only introduced in 4.16-rc2, and so
should be safe assuming it goes in before then.

Reported-by: "Dmitry V. Levin" <ldv@altlinux.org>
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: "Dmitry V. Levin" <ldv@altlinux.org>
Signed-off-by: Kees Cook <keescook@chromium.org>