[XFS] xfssyncd: don't call xfs_sync

Start de-multiplexing xfs_sync() by making xfs_sync_worker() call the
specific sync functions it needs. This is only a small, unique subset of
the entire xfs_sync() code so is easier to follow.

SGI-PV: 988140

SGI-Modid: xfs-linux-melb:xfs-kern:32307a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>