Remove lafs_write_super.

Remove lafs_write_super.

We don't really want to do anything of lafs_write_super
as we write the superblock when needed anyway.
However lafs_sync_fs needs to do what lafs_write_super was doing,
at least sometimes.

lafs_sync_fs will now force a checkpoint exactly when s_dirt is
set.  So revise those setting a little - I think we only want this if
there are dirty inodes to flush. but that needs to be thought about
more when I fix write_inode.

Signed-off-by: NeilBrown <neilb@suse.de>