[sheepdog] [PATCH] object cache: use fcntl() for create_cache_object()

[Christoph Hellwig]

Given that everything else in the object cache uses fcntl locking
this makes sense.

This just leaves my store locking using lockf, which probably also
should switch to fcntl locking.

In addition some little helpers to isolate the calling code from using
struct flock manipulation would be nice.