nfsd: fix callback restarts

Checking the rpc_client pointer is not a reliable way to detect
backchannel changes: cl_cb_client is changed only after shutting down
the rpc client, so the condition cl_cb_client = tk_client will always be
true.

Check the RPC_TASK_KILLED flag instead, and rewrite the code to avoid
the buggy cl_callbacks list and fix the lifetime rules due to double
calls of the ->prepare callback operations method for this retry case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>