nfsd: when CB_SEQUENCE gets ESERVERFAULT don't increment seq_nr

ESERVERFAULT means that the server sent a successful and legitimate
reply, but the session info didn't match what was expected. Don't
increment the seq_nr in that case.

Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/nfsd/nfs4callback.c

···
 		ret = true;
 		break;
 	case -ESERVERFAULT:
-		++session->se_cb_seq_nr[cb->cb_held_slot];
+		/*
+		 * Call succeeded, but the session, slot index, or slot
+		 * sequence number in the response do not match the same
+		 * in the server's call. The sequence information is thus
+		 * untrustworthy.
+		 */
 		nfsd4_mark_cb_fault(cb->cb_clp);
 		break;
 	case 1: