cam/xpt: improve comment on xpt_path_inq

Fix a comment about bzero maybe unnecessary. It can sometimes be
redunant, but a common usage pattern puts the ccb_pathinq structure on
the stack since it's small and for that scenario, it's required. It's
reundant for the few places the ccb is allocated, and in those cases it
does no harm.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D51166