flush writes on close

[ericvh]

9p semantics are such that clunk should trigger a flush of any outstanding write cache (otherwise there might not be a fid to flush with). Change the writeback semantics to always flush on close of the file.

We need two testing regimines to better understand this:

• read/write -- need to make sure we are actually using writeback cache until close, simple testing from shell is hard to do this
• don't interfere with read cache -- we might not want to flush read on close in higher cache modes, so make sure we aren't invalidating the cache just because we are closing the fid.

[ericvh]

fixed f07a6bbd4e8deb8beddc2ce3f61756bdc59e4711