writeback: remove bdi->congested_fn

writeback: remove bdi->congested_fn

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: Christoph Hellwig <[email protected]>
Acked-by: Song Liu <[email protected]>
Acked-by: David Sterba <[email protected]>
[axboe: fixup unused variables in bcache/request.c]
Signed-off-by: Jens Axboe <[email protected]>