nfs: don't retry detect_trunking with RPC_AUTH_UNIX more than once

Currently, when we try to mount and get back NFS4ERR_CLID_IN_USE or
NFS4ERR_WRONGSEC, we create a new rpc_clnt and then try the call again.
There is no guarantee that doing so will work however, so we can end up
retrying the call in an infinite loop.

Worse yet, we create the new client using rpc_clone_client_set_auth,
which creates the new client as a child of the old one. Thus, we can end
up with a *very* long lineage of rpc_clnts. When we go to put all of the
references to them, we can end up with a long call chain that can smash
the stack as each rpc_free_client() call can recurse back into itself.

This patch fixes this by simply ensuring that the SETCLIENTID call will
only be retried in this situation if the last attempt did not use
RPC_AUTH_UNIX.

Note too that with this change, we don't need the (i > 2) check in the
-EACCES case since we now have a more reliable test as to whether we
should reattempt.

Cc: stable@vger.kernel.org # v3.10+
Cc: Chuck Lever <chuck.lever@oracle.com>
Tested-by/Acked-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4state.c

@@ -2093,10 +2093,15 @@ int nfs4_discover_server_trunking(struct nfs_client *clp,
 			nfs4_root_machine_cred(clp);
 			goto again;
 		}
-		if (i > 2)
+		if (clnt->cl_auth->au_flavor == RPC_AUTH_UNIX)
 			break;
 	case -NFS4ERR_CLID_INUSE:
 	case -NFS4ERR_WRONGSEC:
+		/* No point in retrying if we already used RPC_AUTH_UNIX */
+		if (clnt->cl_auth->au_flavor == RPC_AUTH_UNIX) {
+			status = -EPERM;
+			break;
+		}
 		clnt = rpc_clone_client_set_auth(clnt, RPC_AUTH_UNIX);
 		if (IS_ERR(clnt)) {
 			status = PTR_ERR(clnt);