[TIPC]: Fix infinite loop in netlink handler

The tipc netlink config handler uses the nlmsg_pid from the
request header as destination for its reply. If the application
initialized nlmsg_pid to 0, the reply is looped back to the kernel,
causing hangup. Fix: use nlmsg_pid of the skb that triggered the
request.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>