pvcalls-front: properly allocate sk

Don't use kzalloc: it ends up leaving sk->sk_prot not properly
initialized. Use sk_alloc instead and define our own trivial struct
proto.

Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>