Use xpt_path_sbuf() in few drivers

xpt_path_string() is now a wrapper around xpt_path_sbuf(). Using it
to than concatenate result to another sbuf makes no sense. Just call
xpt_path_sbuf() directly.

MFC after: 1 month

(cherry picked from commit 8c4ee0b22c98fc1e208dd133f617bd329cd10728)