fincore: Handle large files correctly on 32 bit

fincore: Handle large files correctly on 32 bit

If a file is larger than 4 GB on a 32 bit system with large file
support (default), it can happen that not all pages are properly
processed. This happens due to an int truncation (off_t vs size_t).

You can reproduce this on 32 bit with these commands:

$ dd if=/dev/zero of=4gb-file seek=4294967295 count=1 bs=1
$ fincore 4gb-file
fincore: failed to do mmap: 4gb-file: Invalid argument

If a file is larger than 4 GB, the first few pages of a file won't
be properly processed. "len" will be smaller than window_size,
but the for-loop iterates "window_size" bytes, skipping some pages.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>