ZTS: zdb_block_size_histogram increase variance

The expected variance for this test case was originally set at 10%
based on local testing.  Additional testing via the CI has show it
can be as large as 11%.  Increase the expected maximum to 12% to
prevent this test from incorrectly failing.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11148

tests/zfs-tests/tests/functional/cli_root/zdb/zdb_block_size_histogram.ksh

@@ -209,11 +209,11 @@ function histo_check_test_pool
 	# 4096 blocksize count for asize.   For verification we stick
 	# to just lsize counts.
 	#
-	# The max_variance is hard-coded here at 10%.  testing so far
-	# has shown this to be in the range of 2%-8% so we leave a
-	# generous allowance... This might need changes in the future
+	# The max_variance is hard-coded here at 12% to leave us some
+	# margin.  Testing has shown this normally to be in the range
+	# of 2%-8%, but it may be as large as 11%.
 	###################
-	let max_variance=10
+	let max_variance=12
 	let fail_value=0
 	let error_count=0
 	log_note "Comparisons for ${pool}"