cam: change xpt_clone_path to return int

xpt_clone_path originally returned a cam_status, but it doesn't do I/O
and should return an errno instead. I added it last year and it's only
used in one place. It's not yet documented, so no doc changes are
nneeded.

Reviewed by: scottl@, mav@
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30884