Change corruption_test to use 4 bits.

Summary:
In the patch which LRU cache was made use dynamic shard bits, I changed to 2 shard bits to make the test happy. Look like it is occasionally still unhappy. Change it to 4 shard bits.
Closes https://github.com/facebook/rocksdb/pull/1815

Differential Revision: D4475849

Pulled By: siying

fbshipit-source-id: 575ff00

db/corruption_test.cc

@@ -44,7 +44,7 @@ class CorruptionTest : public testing::Test {
     // If LRU cache shard bit is smaller than 2 (or -1 which will automatically
     // set it to 0), test SequenceNumberRecovery will fail, likely because of a
     // bug in recovery code. Keep it 4 for now to make the test passes.
-    tiny_cache_ = NewLRUCache(100, 2);
+    tiny_cache_ = NewLRUCache(100, 4);
     options_.wal_recovery_mode = WALRecoveryMode::kTolerateCorruptedTailRecords;
     options_.env = &env_;
     dbname_ = test::TmpDir() + "/corruption_test";