vsftpd: Disable seccomp filtering on 64-bit

It worked on Linux 3.4 but fails with "500 OOPS: priv_sock_get_cmd"
since we updated the default kernel to 3.10.

http://hydra.nixos.org/build/6715359

https://bugzilla.redhat.com/show_bug.cgi?id=845980
https://bugzilla.novell.com/show_bug.cgi?id=786024