NFS: Slow down state manager after an unhandled error

If the state manager thread is not actually able to fully recover from
some situation, it wakes up waiters, who kick off a new state manager
thread.  Quite often the fresh invocation of the state manager is just
as successful.

This results in a livelock as the client dumps thousands of NFS
requests a second on the network in a vain attempt to recover.  Not
very friendly.

To mitigate this situation, add a delay in the state manager after
an unhandled error, so that the client sends just a few requests
every second in this case.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4state.c

@@ -2015,6 +2015,7 @@ static void nfs4_state_manager(struct nfs_client *clp)
 	pr_warn_ratelimited("NFS: state manager%s%s failed on NFSv4 server %s"
 			" with error %d\n", section_sep, section,
 			clp->cl_hostname, -status);
+	ssleep(1);
 	nfs4_end_drain_session(clp);
 	nfs4_clear_state_manager_bit(clp);
 }