def-tool: Replace buf.size() with len(buf).

This commit replaces `buf.size()` with `len(buf)`.  The former
returns the file size and the later returns the mapped buffer size.
`len(buf)` is more general because `buf.size()` is only available to
`mmap` instances.

Test: tests/run.py
Change-Id: I66e8ecd5b475f4fc686aba062b3486ce4ac7f882