netgraph: increase size of sockaddr_ng to match maximum node name

The ng_socket(4) node already writes more than declared size of the
struct at least in the in ng_getsockaddr(). Make size match size of
a node name. The value is pasted instead of including ng_message.h
into ng_socket.h. This is external API and we want to keep it stable
even if NG_NODESIZ is redefined in a kernel build.

Reviewed by: afedorov
Differential Revision: https://reviews.freebsd.org/D42690