Extend zdb to print inconsistencies in livelists and metaslabs

Livelists and spacemaps are data structures that are logs of allocations
and frees.  Livelists entries are block pointers (blkptr_t). Spacemaps
entries are ranges of numbers, most often used as to track
allocated/freed regions of metaslabs/vdevs.

These data structures can become self-inconsistent, for example if a
block or range can be "double allocated" (two allocation records without
an intervening free) or "double freed" (two free records without an
intervening allocation).

ZDB (as well as zfs running in the kernel) can detect these
inconsistencies when loading livelists and metaslab.  However, it
generally halts processing when the error is detected.

When analyzing an on-disk problem, we often want to know the entire set
of inconsistencies, which is not possible with the current behavior.
This commit adds a new flag, `zdb -y`, which analyzes the livelist and
metaslab data structures and displays all of their inconsistencies.
Note that this is different from the leak detection performed by
`zdb -b`, which checks for inconsistencies between the spacemaps and the
tree of block pointers, but assumes the spacemaps are self-consistent.

The specific checks added are:

Verify livelists by iterating through each sublivelists and:
- report leftover FREEs
- report double ALLOCs and double FREEs
- record leftover ALLOCs together with their TXG [see Cross Check]

Verify spacemaps by iterating over each metaslab and:
- iterate over spacemap and then the metaslab's entries in the
  spacemap log, then report any double FREEs and double ALLOCs

Verify that livelists are consistenet with spacemaps.  The space
referenced by livelists (after using the FREE's to cancel out
corresponding ALLOCs) should be allocated, according to the spacemaps.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Sara Hartse <sara.hartse@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
External-issue: DLPX-66031
Closes #10515

man/man8/zdb.8

@@ -10,7 +10,7 @@
 .\"
 .\"
 .\" Copyright 2012, Richard Lowe.
-.\" Copyright (c) 2012, 2018 by Delphix. All rights reserved.
+.\" Copyright (c) 2012, 2019 by Delphix. All rights reserved.
 .\" Copyright 2017 Nexenta Systems, Inc.
 .\" Copyright (c) 2017 Lawrence Livermore National Security, LLC.
 .\" Copyright (c) 2017 Intel Corporation.
@@ -23,7 +23,7 @@
 .Nd display zpool debugging and consistency information
 .Sh SYNOPSIS
 .Nm
-.Op Fl AbcdDFGhikLMPsvXY
+.Op Fl AbcdDFGhikLMPsvXYy
 .Op Fl e Oo Fl V Oc Op Fl p Ar path ...
 .Op Fl I Ar inflight I/Os
 .Oo Fl o Ar var Ns = Ns Ar value Oc Ns ...
@@ -403,6 +403,12 @@ but read transactions otherwise deemed too old.
 Attempt all possible combinations when reconstructing indirect split blocks.
 This flag disables the individual I/O deadman timer in order to allow as
 much time as required for the attempted reconstruction.
+.It Fl y
+Perform validation for livelists that are being deleted.
+Scans through the livelist and metaslabs, checking for duplicate entries
+and compares the two, checking for potential double frees.
+If it encounters issues, warnings will be printed, but the command will not
+necessarily fail.
 .El
 .Pp
 Specifying a display option more than once enables verbosity for only that