xfs: remove wrapper for the fsync file operation

Currently the fsync file operation is divided into a low-level
routine doing all the work and one that implements the Linux file
operation and does minimal argument wrapping.  This is a leftover
from the days of the vnode operations layer and can be removed to
simplify the code a bit, as well as preparing for the implementation
of an optimized fdatasync which needs to look at the Linux inode
state.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>