fs: improve readFile performance

This increases the maximum buffer size per read to 256kb when using
`fs.readFile`. This is important to improve the read performance for
bigger files.

Refs: nodejs#25741

lib/internal/fs/read_file_context.js

@@ -79,7 +79,8 @@ class ReadFileContext {
     } else {
       buffer = this.buffer;
       offset = this.pos;
-      length = Math.min(kReadFileBufferLength, this.size - this.pos);
+      // Read up to 256kb.
+      length = Math.min(256 * 1024, this.size - this.pos);
     }
 
     const req = new FSReqCallback();