The /s3/multipart/create and /s3/entries endpoints interpret relativePath
as an absolute path from the drive root, not relative to parent_id. When
root_folder_id was set to a non-root folder, files larger than
upload_cutoff ended up at the user's drive root instead of the configured
folder.
Resolve the absolute path of the Fs root once via GET /folders/{hash}/path
(cached on first OpenChunkWriter call) and use that to build the correct
relativePath.
Fixes#9392
Nick Craig-Wood
667903dca0