xfs: make sure xfs_sync_fsdata covers the log

We want to always cover the log after writing out the superblock, and
in case of a synchronous writeout make sure we actually wait for the
log to be covered.  That way a filesystem that has been sync()ed can
be considered clean by log recovery.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Alex Elder <aelder@sgi.com>