CAM: Remove return value from xpt_path_sbuf()

It is wrong to call sbuf_len() on third-party sbuf. If that sbuf
has a drain function, it ends up in assertion. But even would it
work, it would return not newly written length, but the full one.
Searching through the sources I don't see this value used.

(cherry picked from commit 6332e0f1a4b34707654d6ae2cd3c1e8799970d0b)