SELinux: requesting no permissions in avc_has_perm_noaudit is a BUG()

This patch turns the case where we have a call into avc_has_perm with no
requested permissions into a BUG_ON.  All callers to this should be in
the kernel and thus should be a function we need to fix if we ever hit
this.  The /selinux/access permission checking it done directly in the
security server and not through the avc, so those requests which we
cannot control from userspace should not be able to trigger this BUG_ON.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen D. Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>