Use more precise stat.st_blocks to account cache size on Unix
instead of file length rounded to a file system block size.
There is no similar way on Windows, so rounding to a cache->bsize is kept.
diff --git a/src/core/ngx_open_file_cache.h b/src/core/ngx_open_file_cache.h
index f56dfcc..638bc5d 100644
--- a/src/core/ngx_open_file_cache.h
+++ b/src/core/ngx_open_file_cache.h
@@ -20,6 +20,7 @@
     ngx_file_uniq_t          uniq;
     time_t                   mtime;
     off_t                    size;
+    off_t                    fs_size;
     off_t                    directio;
     size_t                   read_ahead;