bcache: Alloc holder object before async registration

Allocate holder object (cache or cached_dev) before offloading the
rest of the startup to async work. This will allow us to open the block
block device with proper holder.

Signed-off-by: Jan Kara <jack@suse.cz>
Acked-by: Coly Li <colyli@suse.de>
Reviewed-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://lore.kernel.org/r/20230622164658.12861-1-jack@suse.cz


Signed-off-by: Jens Axboe <axboe@kernel.dk>