linsysfs(5): Fix two unrelated issues
- Swap the order of device_get_ivars with device_get_devclass and devclass name validation. This bug was introduced in r323692.
- Error check device_get_children and free the returned list. This bug was introduced in the original linsysfs commit.
Reported by: Oleg V. Nauman <oleg AT theweb.org.ua>, hselasky (1); hselasky (2)
Reviewed by: hselasky
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12407