make kernel.dmesg_restrict sysctl container-aware

Hi,

The kernel.dmesg_restrict sysctl was added to mainline kernels and backported to RHEL5 and RHEL6 by Red Hat. One of its primary purposes was to protect in-kernel addresses from being shown to non-root users and thus hopefully to make exploitation of certain kinds of kernel bugs more difficult or/and less reliable.

With OpenVZ containers, however, the in-container dmesg is usually empty, and only sometimes it contains useful info: that container's iptables -j LOG output.

The attached patch makes kernel.dmesg_restrict tri-state:

0: no restriction;
1: non-root users can't access dmesg, root users on both hardware node and in containers can access dmesg (seeing different log records as appropriate);
2: non-root users and any user and root in containers can't access dmesg, only root on the hardware node can access dmesg.

2 corresponds to behavior currently seen with 1, whereas the behavior with 1 becomes more relaxed. 1 is then a reasonable default setting for a distro (that's what we use on Owl now).

Please consider applying this change to your currently maintained kernel branches.

Thanks,

Alexander