Fix build of example/memfs_ll.cc on 32 bit architectures

Fix build of example/memfs_ll.cc on 32 bit architectures

The code uses std::min() which expects its arguments to be size_t. Two
times it uses an offset declared as off_t. While both size_t and off_t
are 32-bit integers, the latter is signed. On 64 bit architectures
the conversation of off_t -> size_t performed automatically. On 32 bit
architectures it needs a type cast.

Signed-off-by: Laszlo Boszormenyi (GCS) <gcs@debian.org>