bcache: add bcache_ prefix to btree_root() and btree() macros

This patch changes macro btree_root() and btree() to bcache_btree_root()
and bcache_btree(), to avoid potential generic name clash in future.

NOTE: for product kernel maintainers, this patch can be skipped if
you feel the rename stuffs introduce inconvenince to patch backport.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>