Convert bcachefs to use bdev_open_by_path() and pass the handle around.

CC: Kent Overstreet <kent.overstreet@linux.dev>
CC: Brian Foster <bfoster@redhat.com>
CC: <linux-bcachefs@vger.kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20231101174325.10596-1-jack@suse.czAcked-by: Kent Overstreet <kent.overstreet@linux.dev>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>