sys: Mark ACL conversion routines as __result_use_check

Both acl_copy_oldacl_into_acl() and acl_copy_acl_into_oldacl() may fail
in some circumstances (e.g., acl.acl_cnt exceeding the capacity of
OLDACL_MAX_ENTRIES). This change marks both routines with
__result_use_check, enforcing check for errors by the caller.

Suggested by: markj
Reviewed by: markj, emaste
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46254