Properly account block_decompress_time (#6658)

Summary:
It was incorrectly counting time even for blocks that didn't need decompression.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6658

Test Plan: make check

Reviewed By: ajkr

Differential Revision: D20883522

Pulled By: anand1976

fbshipit-source-id: 33c9c4683f54cad150ab260a69e3ef8aa9aff76a

table/block_fetcher.cc

@@ -260,11 +260,10 @@ Status BlockFetcher::ReadBlockContents() {
     }
   }
 
-  PERF_TIMER_GUARD(block_decompress_time);
-
   compression_type_ = get_block_compression_type(slice_.data(), block_size_);
 
   if (do_uncompress_ && compression_type_ != kNoCompression) {
+    PERF_TIMER_GUARD(block_decompress_time);
     // compressed page, uncompress, update cache
     UncompressionContext context(compression_type_);
     UncompressionInfo info(context, uncompression_dict_, compression_type_);