ztest: split block reconstruction

Increase the default allowed number of reconstruction attempts.
There's not an exact right number for this setting.  It needs
to be set large enough to cover any realistic failure scenarios
and small enough to avoid stalling the IO pipeline and invoking
the dead man detection.

The current value of 256 was empirically determined to be too
low based on multi-day runs of ztest.  The fault injection code
would inject more damage than could be reconstructed given the
relatively small number of attempts.  However, in all observed
cases the block could be reconstructed using a slightly higher
limit.

Based on local testing increasing the default value to 4096 was
determined to strike the best balance.  Checking all combinations
takes less than 10s in the worst case, and has so far eliminated
the vast majority of false positives detected by ztest.  This
delay is roughly on par with how long retries may be performed
to a misbehaving HDD and was deemed to be reasonable.  Better to
err on the side of a brief delay rather than fail to reconstruct
the data.

Lastly, the -Y flag has been added to zdb to make it easy to try all
possible combinations when performing split block reconstruction.
For badly damaged blocks with 18 splits, they can be fully enumerated
within a few minutes.  This has been done to ensure permanent errors
are never incorrectly reported when ztest verifies the pool with zdb.

Reviewed by: Tom Caputi <tcaputi@datto.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8271

man/man5/zfs-module-parameters.5

@@ -2025,7 +2025,7 @@ combinations each time the block is accessed.  This allows all segment
 copies to participate fairly in the reconstruction when all combinations
 cannot be checked and prevents repeated use of one bad copy.
 .sp
-Default value: \fB256\fR.
+Default value: \fB4096\fR.
 .RE
 
 .sp

man/man8/zdb.8

@@ -23,7 +23,7 @@
 .Nd display zpool debugging and consistency information
 .Sh SYNOPSIS
 .Nm
-.Op Fl AbcdDFGhikLMPsvX
+.Op Fl AbcdDFGhikLMPsvXY
 .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 ...
@@ -50,7 +50,7 @@
 .Ar device
 .Nm
 .Fl m
-.Op Fl AFLPX
+.Op Fl AFLPXY
 .Op Fl e Oo Fl V Oc Op Fl p Ar path ...
 .Op Fl t Ar txg
 .Op Fl U Ar cache
@@ -349,6 +349,10 @@ Attempt
 transaction rewind, that is attempt the same recovery as
 .Fl F
 but read transactions otherwise deemed too old.
+.It Fl Y
+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.
 .El
 .Pp
 Specifying a display option more than once enables verbosity for only that