bcache: fix a regression of code compiling failure in debug.c

bcache: fix a regression of code compiling failure in debug.c

The patch "bcache: remove PTR_CACHE" introduces a compiling failure in
debug.c with following error message,
  In file included from drivers/md/bcache/bcache.h:182:0,
                   from drivers/md/bcache/debug.c:9:
  drivers/md/bcache/debug.c: In function 'bch_btree_verify':
  drivers/md/bcache/debug.c:53:19: error: 'c' undeclared (first use in
  this function)
    bio_set_dev(bio, c->cache->bdev);
                     ^
This patch fixes the regression by replacing c->cache->bdev by b->c->
cache->bdev.

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210411134316.80274-8-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>