selinux: Fix regression for Xorg

Commit 6f5317e730505d5cbc851c435a2dfe3d5a21d343 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" <[e-mail]>
Signed-off-by: Stephen Smalley <[e-mail]>
Signed-off-by: James Morris <[e-mail]>

security/selinux/ss/services.c

···
 			return i;
 	}
 
-	return pol_value;
+	return SECCLASS_NULL;
 }
 
 static void map_decision(u16 tclass, struct av_decision *avd,