netfilter: ipset: Rework long task execution when adding/deleting entries

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

netfilter: ipset: Rework long task execution when adding/deleting entries

When adding/deleting large number of elements in one step in ipset, it can

take a reasonable amount of time and can result in soft lockup errors. The

patch 5f7b51bf09ba ("netfilter: ipset: Limit the maximal range of

consecutive elements to add/delete") tried to fix it by limiting the max

elements to process at all. However it was not enough, it is still possible

that we get hung tasks. Lowering the limit is not reasonable, so the

approach in this patch is as follows: rely on the method used at resizing

sets and save the state when we reach a smaller internal batch limit,

unlock/lock and proceed from the saved state. Thus we can avoid long

continuous tasks and at the same time removed the limit to add/delete large

number of elements in one step.

The nfnl mutex is held during the whole operation which prevents one to

issue other ipset commands in parallel.