orangefs: rename remaining bits of mmap readahead cache

This has been dormant code for many years. Parts of it were removed from
the OrangeFS kernel code when it went into mainline. These bits were missed.
Now the readahead cache has been resurrected in the OrangeFS userspace
portions. It was renamed there, since it doesn't really have anything to do
with mmap specifically, so it will be renamed here.

Signed-off-by: Martin Brandenburg <martin@omnibond.com>