xfs: split inode data writeback from xfs_sync_inodes_ag

In many cases we only want to sync inode data. Start spliting the inode sync
into data sync and inode sync by factoring out the inode data flush.

[hch: minor cleanups]

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