nfsd: move callback rpc_client creation into separate thread

The whole reason to move this callback-channel probe into a separate
thread was because (for now) we don't have an easy way to create the
rpc_client asynchronously.  But I forgot to move the rpc_create() to the
spawned thread.  Doh!  Fix that.

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