cp: Fix improper use of O_PATH.

This does not appear to make any practical difference at the moment, but
technically O_PATH means “I'm not going to use this descriptor for any
other purposes than vnode lookups”, so using it to read the directory's
ACLs is improper and might fail in the future.

Fixes: 82fc0d09e8625
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D50984