The rfc allows a client to change the callback parameters, but we didn't
previously implement it.
Teach the callbacks to rerun themselves (by placing themselves on a
workqueue) when they recognize that their rpc task has been killed and
that the callback connection has changed.
Then we can change the callback connection by setting up a new rpc
client, modifying the nfs4 client to point at it, waiting for any work
in progress to complete, and then shutting down the old client.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
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