Let's start saying that your job is awesome. I found it really helful!!
I am working on kernel 2.4 andI modified a little bit your APIs.
I think you might find helpful these few notes.

On kclose function:
Line 228: ret = sk->ops->release(sk);
is not necessary since sock_release already do that.  Moreover, it would lead to a segmentation fault if sk is already closed.

kclose does not set the socket pointer to NULL. This mean that if you kclose it again, it will result in a segmentation fault.
It is important because the check if(sk) will never fail on a socket even if it was closed.

Same problem come out with kfree. I suggest to implement a kfree that also set the pointer to NULL.

If you need any clarification or any help, just contact me :D

See you.