Optimize possible split block search space

Remove duplicate segment copies to minimize the possible search
space for reconstruction.  Once reduced an accurate assessment can
be made regarding the difficulty in reconstructing the block.

Also, ztest will now run zdb with
zfs_reconstruct_indirect_combinations_max set to 1000000 in an attempt
to avoid checksum errors.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6900

man/man5/zfs-module-parameters.5

@@ -1742,18 +1742,18 @@ Use \fB1\fR for yes and \fB0\fR for no (default).
 .sp
 .ne 2
 .na
-\fBzfs_reconstruct_indirect_segments_max\fR (int)
+\fBzfs_reconstruct_indirect_combinations_max\fR (int)
 .ad
-.RS 12n
-When a split block which is part of a indirect vdev contains more than this
-many segments, consider it too computationally expensive to check all possible
-combinations.  Instead, operate under the assumption that only a few segment
-copies are damaged and the majority of segment copies are good, in which case
-it is reasonable to randomly select sample combinations.  This allows all the
-segment copies to participate fairly in the reconstruction and prevents the
-repeated use of one bad copy.
+.RS 12na
+If an indirect split block contains more than this many possible unique
+combinations when being reconstructed, consider it too computationally
+expensive to check them all. Instead, try at most
+\fBzfs_reconstruct_indirect_combinations_max\fR randomly-selected
+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: \fB10\fR.
+Default value: \fB100\fR.
 .RE
 
 .sp