seccomp: add arch_prctl() to the syscall whitelist

It appears that even a very simple /etc/qemu-ifup configuration can
require the arch_prctl() syscall, see the example below:

	#!/bin/sh
	/sbin/ifconfig $1 0.0.0.0 up
	/usr/sbin/brctl addif <switch> $1

Signed-off-by: Paul Moore <pmoore@redhat.com>
Reviewed-by: Eduardo Otubo <otubo@linux.vnet.ibm.com>
Message-id: 20130718135703.8247.19213.stgit@localhost
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

qemu-seccomp.c

@@ -217,7 +217,8 @@ static const struct QemuSeccompSyscall seccomp_whitelist[] = {
     { SCMP_SYS(waitid), 241 },
     { SCMP_SYS(io_cancel), 241 },
     { SCMP_SYS(io_setup), 241 },
-    { SCMP_SYS(io_destroy), 241 }
+    { SCMP_SYS(io_destroy), 241 },
+    { SCMP_SYS(arch_prctl), 240 }
 };
 
 int seccomp_start(void)