net: Relax xmalloc-ing (and fix NULL deref)

There's potential NULL-derefernece in dump_netns_con() -- two xmalloc
results are not checked. However, since there's a huge set of these
xmallocs, I propose to relax the whole thing with one big xmalloc and
xptr_pull() helper.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>