nfsd4: rename callback struct to cb_conn

I want to use the name for a struct that actually does represent a
single callback.

(Actually, I've never been sure it helps to a separate struct for the
callback information.  Some day maybe those fields could just be dumped
into struct nfs4_client.  I don't know.)

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>