seccomp: block use of clone3 syscall

seccomp: block use of clone3 syscall

Modern glibc will use clone3 instead of clone, when it detects that it
is available. We need to compare flags in order to decide whether to
allow clone (thread create vs process fork), but in clone3 the flags
are hidden inside a struct. Seccomp can't currently match on data inside
a struct, so our only option is to block clone3 entirely. If we use
ENOSYS to block it, then glibc transparently falls back to clone.

This may need to be revisited if Linux adds a new architecture in
future and only provides clone3, without clone.

Acked-by: Eduardo Otubo <otubo@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>