Interface group names must fit into IFNAMSIZ and be unique. But

Interface group names must fit into IFNAMSIZ and be unique.  But
the kernel made the unique check before trunkating with strlcpy().
So there could be two interface groups with the same name.  The kif
is created by a name lookup.  The trunkated names are equal, so
there was only one kif owned by both groups.  When the groups got
destroyed, the single kif was removed twice from the RB tree.
Check length of group name before doing the unique check.
The empty group name was allowed and is now invalid.
Reported-by: syzbot+f47e8296ebd559f9bbff@syzkaller.appspotmail.com
OK deraadt@ gnezdo@ anton@ mvs@ claudio@