[issue4681] mmap offset should be off_t instead of ssize_t, and size calculation needs corrected

[Antoine Pitrou]

Changes by Antoine Pitrou <pitrou at free.fr>:


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stage:  -> patch review
versions: +Python 3.2, Python 3.3 -Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue4681>
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