Advise: When map_entir_file, the len of file to map including the offset may be more appropriate

[zhangzidan]

Just as the title:
in some condition, we need map entire file from the offset.

In addition, if offset>0, map the entire file length is not appropriate。

I modified the corresponding file locally：

[vimpunk]

@aijyo Oops, that's a bug indeed. Thanks a lot for filing this!

And apologies for the late reply. GH doesn't send me notifications for some reason :(