selinux: Fix regression for Xorg

Commit 6f5317e introduced a bug in the
handling of userspace object classes that is causing breakage for Xorg
when XSELinux is enabled.  Fix the bug by changing map_class() to return
SECCLASS_NULL when the class cannot be mapped to a kernel object class.

Reported-by:  "Justin P. Mattock" <justinmattock@gmail.com>
Signed-off-by:  Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>

security/selinux/ss/services.c

@@ -213,7 +213,7 @@ static u16 map_class(u16 pol_value)
 			return i;
 	}
 
-	return pol_value;
+	return SECCLASS_NULL;
 }
 
 static void map_decision(u16 tclass, struct av_decision *avd,