block: prohibit calls to bio_chain_endio

Now that all potential callers of bio_chain_endio have been
eliminated, completely prohibit any future calls to this function.

Suggested-by: Ming Lei <[e-mail]>
Suggested-by: Andreas Gruenbacher <[e-mail]>
Suggested-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Shida Zhang <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/bio.c

···
 	return parent;
 }
 
+/*
+ * This function should only be used as a flag and must never be called.
+ * If execution reaches here, it indicates a serious programming error.
+ */
 static void bio_chain_endio(struct bio *bio)
 {
-	bio_endio(__bio_chain_endio(bio));
+	BUG();
 }
 
 /**