bcache: move uapi header bcache.h to bcache code directory

The header file include/uapi/linux/bcache.h is not really a user space
API heaer. This file defines the ondisk format of bcache internal meta
data but no one includes it from user space, bcache-tools has its own
copy of this header with minor modification.

Therefore, this patch moves include/uapi/linux/bcache.h to bcache code
directory as drivers/md/bcache/bcache_ondisk.h.

Suggested-by: Arnd Bergmann <arnd@kernel.org>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Coly Li <colyli@suse.de>
Link: https://lore.kernel.org/r/20211029060930.119923-2-colyli@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>