orangefs: avoid fsync service operation on flush

Without this, an fsync call is sent to the server even if no data
changed.  This resulted in a rather severe (50%) performance regression
under certain metadata-heavy workloads.

In the past, everything was direct IO.  Nothing happend on a close call.
An explicit fsync call would send an fsync request to the server which
in turn fsynced the underlying file.

Now there are cached writes.  Then fsync began writing out dirty pages
in addition to making an fsync request to the server, and close began
calling fsync.

With this commit, close only writes out dirty pages, and does not make
the fsync request.

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