Enhancement request: allow vectored buffers with ReplyData

[asomers]

It's possible that a FUSE server may store file data in discrete pieces. Ideally the data would only need to be copied once, when writing to /dev/fuse with writev. But fuse3's ReplyData struct cannot accept vectored data. Instead, it forces the file system to do an extra data copy if the data is vectored.

As an enhancement, it would be great if fuse3 had a way for read to return vectored data.