[TIPC]: Improve response to requests for node/link information

Now allocates reply space for "get links" request based on number of actual
links, not number of potential links.  Also, limits reply to "get links" and
"get nodes" requests to 32KB to match capabilities of tipc-config utility
that issued request.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Per Liden <per.liden@ericsson.com>