b2: fix chunked streaming uploads

Streaming uploads are used by rclone rcat and rclone mount
--vfs-cache-mode off.

After the multipart chunker refactor the multipart chunked streaming
upload was accidentally mixing the first and the second parts up which
was causing corrupted uploads.

This was caused by a simple off by one error in the refactoring where
we went from 1 based part number counting to 0 based part number
counting.

Fixing this revealed that the metadata wasn't being re-read for the
copied object either.

This fixes both of those issues and adds an integration tests so it
won't happen again.

Fixes #7367

backend/b2/upload.go

@@ -393,10 +393,11 @@ func (up *largeUpload) Stream(ctx context.Context, initialUploadBlock *pool.RW)
 		hasMoreParts = true
 	)
 	up.size = initialUploadBlock.Size()
+	up.parts = 0
 	for part := 0; hasMoreParts; part++ {
 		// Get a block of memory from the pool and token which limits concurrency.
 		var rw *pool.RW
-		if part == 1 {
+		if part == 0 {
 			rw = initialUploadBlock
 		} else {
 			rw = up.f.getRW(false)
@@ -411,7 +412,7 @@ func (up *largeUpload) Stream(ctx context.Context, initialUploadBlock *pool.RW)
 
 		// Read the chunk
 		var n int64
-		if part == 1 {
+		if part == 0 {
 			n = rw.Size()
 		} else {
 			n, err = io.CopyN(rw, up.in, up.chunkSize)
@@ -426,7 +427,7 @@ func (up *largeUpload) Stream(ctx context.Context, initialUploadBlock *pool.RW)
 		}
 
 		// Keep stats up to date
-		up.parts = part
+		up.parts += 1
 		up.size += n
 		if part > maxParts {
 			up.f.putRW(rw)