OpenZFS 8063 - verify that we do not attempt to access inactive txg

Authored by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: George Melikov <mail@gmelikov.ru>

A standard practice in ZFS is to keep track of "per-txg" state. Any of
the 3 active TXG's (open, quiescing, syncing) can have different values
for this state. We should assert that we do not attempt to modify other
(inactive) TXG's.

Porting Notes:
- ASSERTV added to txg_sync_waiting() for unused variable.

OpenZFS-issue: https://www.illumos.org/issues/8063
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/01acb46
Closes #6109

include/sys/zil.h

@@ -20,7 +20,7 @@
  */
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2017 by Delphix. All rights reserved.
  */
 
 /* Portions Copyright 2010 Robert Milkowski */
@@ -94,6 +94,15 @@ typedef struct zil_chain {
 
 #define	ZIL_MIN_BLKSZ	4096ULL
 
+/*
+ * ziltest is by and large an ugly hack, but very useful in
+ * checking replay without tedious work.
+ * When running ziltest we want to keep all itx's and so maintain
+ * a single list in the zl_itxg[] that uses a high txg: ZILTEST_TXG
+ * We subtract TXG_CONCURRENT_STATES to allow for common code.
+ */
+#define	ZILTEST_TXG (UINT64_MAX - TXG_CONCURRENT_STATES)
+
 /*
  * The words of a log block checksum.
  */