netfilter: nf_conncount: update last_gc only when GC has been performed

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_conncount: update last_gc only when GC has been performed

Currently last_gc is being updated everytime a new connection is

tracked, that means that it is updated even if a GC wasn't performed.

With a sufficiently high packet rate, it is possible to always bypass

the GC, causing the list to grow infinitely.

Update the last_gc value only when a GC has been actually performed.