sysctl: restrict write access to dmesg_restrict

When dmesg_restrict is set to 1 CAP_SYS_ADMIN is needed to read the kernel
ring buffer.  But a root user without CAP_SYS_ADMIN is able to reset
dmesg_restrict to 0.

This is an issue when e.g.  LXC (Linux Containers) are used and complete
user space is running without CAP_SYS_ADMIN.  A unprivileged and jailed
root user can bypass the dmesg_restrict protection.

With this patch writing to dmesg_restrict is only allowed when root has
CAP_SYS_ADMIN.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Dan Rosenberg <drosenberg@vsecurity.com>
Acked-by: Serge E. Hallyn <serge@hallyn.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: Kees Cook <kees.cook@canonical.com>
Cc: James Morris <jmorris@namei.org>
Cc: Eugene Teo <eugeneteo@kernel.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>