Except for pktdvd, the only places setting congested bits are file
systems that allocate their own backing_dev_info structures.  And
pktdvd is a deprecated driver that isn't useful in stack setup
either.  So remove the dead congested_fn stacking infrastructure.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Acked-by: NSong Liu <song@kernel.org>
Acked-by: NDavid Sterba <dsterba@suse.com>
[axboe: fixup unused variables in bcache/request.c]
Signed-off-by: NJens Axboe <axboe@kernel.dk>