XFS: clean up sync code

xfs_sync and xfs_syncsub are multiplexed interfaces that shares relatively little code between callers. Because it is a multiplexed interface, it's hard to tell what is executed in each context it is called.

Factor out the sync code and explicitly call the sync functions needed rather than the multiplexed interfaces. Once this is done, we can remove xfs_syncsub and xfs_sync altogether.