Fix a race where kthread_stop() may prevent the threadfn from ever getting
called.  If that happens the svc_rqst will not be cleaned up.

Fixes: ed6473dd ("NFSv4: Fix callback server shutdown")
Signed-off-by: NBenjamin Coddington <bcodding@redhat.com>
Reviewed-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>