affs: add ->sync_fs

Add a ->sync_fs method for data integrity syncs.  Factor out common code
between affs_put_super, affs_write_super and the new affs_sync_fs into
a helper.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>