knfsd: don't shutdown callbacks until nfsv4 client is freed

If a callback still holds a reference on the client, then it may be
about to perform an rpc call, so it isn't safe to call rpc_shutdown().
(Though rpc_shutdown() does wait for any outstanding rpc's, it can't
know if a new rpc is about to be issued with that client.)

So, wait to shutdown the rpc_client until the reference count on the
client has gone to zero.

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