Used long instead of int for the mmap file size.

Mmapping of 2GB+ files is failing.
This pull request fixes that by using `long` to hold the file size.

Example failing code:

    require 'torch'
    local storage = torch.ByteStorage("more_than_2GB.bin")
    print("storage:size():", storage:size())
    print("storage[1]:", storage[1])