bcache: use bio cloning for detached device requests

bcache: use bio cloning for detached device requests

Previously, bcache hijacked the bi_end_io and bi_private fields of
the incoming bio when the backing device was in a detached state.
This is fragile and breaks if the bio is needed to be processed by
other layers.

This patch transitions to using a cloned bio embedded within a private
structure. This ensures the original bio's metadata remains untouched.

Fixes: 53280e398471 ("bcache: fix improper use of bi_end_io")
Co-developed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Shida Zhang <zhangshida@kylinos.cn>
Acked-by: Coly Li <colyli@fnnas.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>