lagg: fix lagg ifioctl after SIOCSIFCAPNV

Lagg was broken by SIOCSIFCAPNV when all underlying devices
support SIOCSIFCAPNV. This change updates lagg to work with
SIOCSIFCAPNV and if_capabilities2.

Reviewed by: kib, hselasky
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35865