benchmark.sh: to fix divide by zero runtime error (#4442)

Summary:
"Write (GB)" of $9 rather than "Rnp1 (GB)" of $8
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4442

Differential Revision: D10318193

Pulled By: yiwu-arbug

fbshipit-source-id: 03a7ef1938d9332e06fb3fd8490ca212f61fac6b

tools/benchmark.sh

@@ -151,8 +151,8 @@ function summarize_result {
   stall_pct=$( grep "^Cumulative stall" $test_out| tail -1  | awk '{  print $5 }' )
   ops_sec=$( grep ^${bench_name} $test_out | awk '{ print $5 }' )
   mb_sec=$( grep ^${bench_name} $test_out | awk '{ print $7 }' )
-  lo_wgb=$( grep "^  L0" $test_out | tail -1 | awk '{ print $8 }' )
+  lo_wgb=$( grep "^  L0" $test_out | tail -1 | awk '{ print $9 }' )
-  sum_wgb=$( grep "^ Sum" $test_out | tail -1 | awk '{ print $8 }' )
+  sum_wgb=$( grep "^ Sum" $test_out | tail -1 | awk '{ print $9 }' )
   sum_size=$( grep "^ Sum" $test_out | tail -1 | awk '{ printf "%.1f", $3 / 1024.0 }' )
   wamp=$( echo "scale=1; $sum_wgb / $lo_wgb" | bc )
   wmb_ps=$( echo "scale=1; ( $sum_wgb * 1024.0 ) / $uptime" | bc )