Simplify MemTable::Update

Summary:
As suggested by testn in #1650

The Add is at the end of the function. Having a fallthough
will result in it being added twice.
Closes https://github.com/facebook/rocksdb/pull/1676

Differential Revision: D4331906

Pulled By: yiwu-arbug

fbshipit-source-id: 895c4a0

db/memtable.cc

@@ -717,8 +717,7 @@ void MemTable::Update(SequenceNumber seq,
       ValueType type;
       SequenceNumber unused;
       UnPackSequenceAndType(tag, &unused, &type);
-      switch (type) {
-        case kTypeValue: {
+      if (type == kTypeValue) {
         Slice prev_value = GetLengthPrefixedSlice(key_ptr + key_length);
         uint32_t prev_size = static_cast<uint32_t>(prev_value.size());
         uint32_t new_size = static_cast<uint32_t>(value.size());
@@ -735,13 +734,6 @@ void MemTable::Update(SequenceNumber seq,
           return;
         }
       }
-	// fallthrough
-        default:
-          // If the latest value is kTypeDeletion, kTypeMerge or kTypeLogData
-          // we don't have enough space for update inplace
-            Add(seq, kTypeValue, key, value);
-            return;
-      }
     }
   }