benches: Added disk usage measurements to bench_wt

Note bench_wt_seq's disk usage is garbage because of the repeated
truncates!

But I figured this is at least useful for the other benches, and we
already have bench_helper_usage. Maybe in the future we'll figure out
some way to get useful disk usage from bench_wt_seq.

benches/bench_wt.toml

@@ -90,6 +90,12 @@ code = '''
     BENCH_RESULT("heap", written, BENCH_HEAP());
     #endif
 
+    // find the total disk usage after the benchmark
+    //
+    // note this is garbage because of the above truncates!
+    uintmax_t usage = bench_helpers_usage(&lfs3);
+    BENCH_RESULT("usage", written, usage);
+
     lfs3_unmount(&lfs3) => 0;
 '''
 
@@ -145,6 +151,10 @@ code = '''
     BENCH_RESULT("heap", written, BENCH_HEAP());
     #endif
 
+    // find the total disk usage after the benchmark
+    uintmax_t usage = bench_helpers_usage(&lfs3);
+    BENCH_RESULT("usage", written, usage);
+
     lfs3_unmount(&lfs3) => 0;
 '''
 
@@ -220,6 +230,10 @@ code = '''
     BENCH_RESULT("heap", written, BENCH_HEAP());
     #endif
 
+    // find the total disk usage after the benchmark
+    uintmax_t usage = bench_helpers_usage(&lfs3);
+    BENCH_RESULT("usage", written, usage);
+
     lfs3_unmount(&lfs3) => 0;
 '''
 
@@ -285,6 +299,10 @@ code = '''
     BENCH_RESULT("heap", written, BENCH_HEAP());
     #endif
 
+    // find the total disk usage after the benchmark
+    uintmax_t usage = bench_helpers_usage(&lfs3);
+    BENCH_RESULT("usage", written, usage);
+
     lfs3_unmount(&lfs3) => 0;
 '''