selinux: allow multiple opens of /sys/fs/selinux/policy

In the Linux kernel, the following vulnerability has been resolved: selinux: allow multiple opens of /sys/fs/selinux/policy Currently there can only be a single open of /sys/fs/selinux/policy at any time. This allows any process to block any other process from reading the kernel policy. The original motivation seems to have been a mix of preventing an inconsistent view of the policy size and preventing userspace from allocating kernel memory without bound, but this is arguably equally bad. Eliminate the policy_opened flag and shrink the critical section that the policy mutex is held. While we are making changes here, drop a couple of extraneous BUG_ONs.