SELinux: no BUG_ON(!ss_initialized) in selinux_clone_mnt_opts

The Fedora installer actually makes multiple NFS mounts before it loads
selinux policy.  The code in selinux_clone_mnt_opts() assumed that the
init process would always be loading policy before NFS was up and
running.  It might be possible to hit this in a diskless environment as
well, I'm not sure.  There is no need to BUG_ON() in this situation
since we can safely continue given the circumstances.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>