simplify procfs code for seq_file instances V3

[Al Viro]

On Wed, May 16, 2018 at 11:43:04AM +0200, Christoph Hellwig wrote:
> We currently have hundreds of proc files that implement plain, read-only
> seq_file based interfaces.  This series consolidates them using new
> procfs helpers that take the seq_operations or simple show callback
> directly.
> 
> A git tree is available at:
> 
>     git://git.infradead.org/users/hch/misc.git proc_create.3

Pulled, but the last bit is a bleeding atrocity in need of followup
cleanup.