passthrough_hp is the high-performance loopback filesystem. By
exercising it, we can distinguish between kernel bugs (shared between
go-fuse and libfuse) and go-fuse bugs.

Passthrough mode does not work with iflag=direct, because
passthrough_hp does not check if the file is marked "directio",
assuming read() calls in passthrough mode are some kind of error.

Change-Id: I2b6b4c943031e1a2864e15e5bf396aac28c63e31