selinux: replace some BUG_ON()s with a WARN_ON()

We don't need to crash the machine in these cases. Let's just detect the
buggy state early and error out with a warning.

Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Reviewed-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>