fincore: Handle large files on 32 bit without LFS

fincore: Handle large files on 32 bit without LFS

If util-linux is installed on a system without large file support,
an out of memory issue can occur while processing a file which is
2 GB in size:

$ ./configure --disable-largefile && make

$ dd if=/dev/zero of=2gb-file seek=2147483646 count=1 bs=1
$ fincore 2gb-file
(endless loop)
fincore: failed to do mmap: 2gb-file: Cannot allocate memory

Even though iterating with "len" seems counter-intuitive, it fixes
this issue. The variable len is only in the last iteration not a
multiplication of pagesize -- which is the requirement for mmap.

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