xfs: stop calling filemap_fdatawait inside ->fsync

Now that the VFS actually waits for the data I/O to complete before
calling into ->fsync we can stop doing it ourselves.

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