[XFS] kill xfs_syncsub

Now that the only caller is xfs_sync(), merge the two together as it makes
no sense to keep them separate.

SGI-PV: 988140

SGI-Modid: xfs-linux-melb:xfs-kern:32306a

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