userns: Add a Kconfig option to enforce strict kuid and kgid type checks

Make it possible to easily switch between strong mandatory
type checks and relaxed type checks so that the code can
easily be tested with the type checks and then built
with the strong type checks disabled so the resulting
code can be used.

Require strong mandatory type checks when enabling the user namespace.
It is very simple to make a typo and use the wrong type allowing
conversions to/from userspace values to be bypassed by accident,
the strong type checks prevent this.

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/linux/uidgid.h

@@ -17,7 +17,7 @@
 struct user_namespace;
 extern struct user_namespace init_user_ns;
 
-#if defined(NOTYET)
+#ifdef CONFIG_UIDGID_STRICT_TYPE_CHECKS
 
 typedef struct {
 	uid_t val;