net: Validate interface group names in ioctl handlers

The handlers were not checking that the group names are nul-terminated.
Add checks for this.

Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Reviewed by: zlei
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53344

(cherry picked from commit 32919a34f17ac1af99dec7376f22a8393c251602)