The kref now does most of what cl_count + cl_user used to do. The only
remaining role for cl_count is to tell us if we are in a 'shutdown'
phase. We can provide that information using a single bit field instead
of a full atomic counter.

Also rename rpc_destroy_client() to rpc_close_client(), which reflects
better what its role is these days.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>