nfs: do not start the callback thread until we set rqstp->rq_task
nfs: do not start the callback thread until we set rqstp->rq_task

This fixes an Oopsable race when starting up the callback server.

Signed-off-by: Trond Myklebust <cc958843532c615cbd4920e868f78c7985331a5a@primarydata.com>
Reviewed-by: Jeff Layton <4376fbf8623cd7b7c0232225289bb91cedc0a27f@primarydata.com>
Signed-off-by: J. Bruce Fields <51738506c1b2ccb0761f23bdc612c93babf738ea@redhat.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs