[XFS] move xfssyncd code to xfs_sync.c

Move all the xfssyncd code to the new xfs_sync.c file. This places it
closer to the actual code that it interacts with, rather than just being
associated with high level VFS code.

SGI-PV: 988139

SGI-Modid: xfs-linux-melb:xfs-kern:32283a

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