BUG: Double mount silently disables NFSv4 native labeling

[stephensmalley]

Mounting a security_label exported NFS4 filesystem twice silently disables per-file labeling. This is a return of the bug previously fixed by kernel commit 3815a24 ("security/selinux: fix SECURITY_LSM_NATIVE_LABELS on reused superblock"). Until this is fixed, we can't fix the testsuite test of this bug, see SELinuxProject/selinux-testsuite#92