Stress test to allow memtable whole key filter (#7937)

Summary:
Right now, stress test cannot be configured to use memtable whole key filter without prefix filter. It doesn't appear to be necessary. remove this constraint.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7937

Test Plan: "make crash_test" to be able to run.

Reviewed By: ltamasi

Differential Revision: D26295532

fbshipit-source-id: 30c874a9dc2b672a460603a4ee32368674e0face

db_stress_tool/db_stress_tool.cc

@@ -153,10 +153,11 @@ int db_stress_tool(int argc, char** argv) {
             "test_batches_snapshots test!\n");
     exit(1);
   }
-  if (FLAGS_memtable_prefix_bloom_size_ratio > 0.0 && FLAGS_prefix_size < 0) {
+  if (FLAGS_memtable_prefix_bloom_size_ratio > 0.0 && FLAGS_prefix_size < 0 &&
+      !FLAGS_memtable_whole_key_filtering) {
     fprintf(stderr,
-            "Error: please specify positive prefix_size in order to use "
-            "memtable_prefix_bloom_size_ratio\n");
+            "Error: please specify positive prefix_size or enable whole key "
+            "filtering in order to use memtable_prefix_bloom_size_ratio\n");
     exit(1);
   }
   if ((FLAGS_readpercent + FLAGS_prefixpercent + FLAGS_writepercent +