comms/opencbm-kmod: Add error handling for copyin() and copyout() calls

This will be required soon as my intent is to annotate copyin() and
related functions with __result_use_check. Moreover, the lack of error
handling here is a real bug.

The patch was submitted upstream as well.

PR: 275986
Approved by: zirias
Differential Revision: https://reviews.freebsd.org/D43273