There is two variables named 'len' in rtnl_talk. In fact, commit
c079e121 didn't work. For example, it was possible to trigger
a seg fault with this command:
$ ip link set gre2 type ip6gre hoplimit 32

Let's rename the argument len to maxlen.

Fixes: c079e121 ("libnetlink: add size argument to rtnl_talk")
Reported-by: Thomas Faivre <thomas.faivre@6wind.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>