netns: Fail conspicously if someone uses net_generic at an inappropriate time.

By definition net_generic should never be called when it can return
NULL.  Fail conspicously with a BUG_ON to make it clear when people mess
up that a NULL return should never happen.

Recently there was a bug in the CAIF subsystem where it was registered
with register_pernet_device instead of register_pernet_subsys.  It was
erroneously concluded that net_generic could validly return NULL and
that net_assign_generic was buggy (when it was just inefficient).
Hopefully this BUG_ON will prevent people to coming to similar erroneous
conclusions in the futrue.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Tested-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/generic.h

@@ -41,6 +41,7 @@ static inline void *net_generic(const struct net *net, int id)
 	ptr = ng->ptr[id - 1];
 	rcu_read_unlock();
 
+	BUG_ON(!ptr);
 	return ptr;
 }
 #endif