cam: Create free_scan_info helper function

We free the scsi bug scan info in 4 places. 1 was wrong until I fixed a
bug there in a recent commit. Introduce a helper function that will free
the cpi always (it must always be valid since we set it right after
allocation).

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D51170