NFT: name lenghts

[mmetc]

tldr: With the default names "crowdsec-blacklists" and "crowdsec6-blacklists" the bouncer fails to create sets on some systems.

A PR is available with a very limited scope: it suggests the user to change name, but it does not check if the issue comes instead from table or chain names.

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The names of tables, chains and sets have limits. They have historically been at least 32 characters (since at least a few years) then raised to 256 in recent kernels - torvalds/linux@3874549

Now, if a name exceeds the limit we get an error from netlink, like

conn.Receive: netlink receive: numerical result out of range

and we flush all requests together (create table, set, chains) but the error we receive ( https://github.com/google/nftables/blob/main/conn.go#L233 ) does not have the context. We don't know which object has a problem.

This would normally not be an issue but on some systems, the error is raised when the set name is > 15 characters. Example: ubuntu 16.04

The issue can be replicated on the command line:

$ sudo nft add set ip crowdsec crowdsec-blackli { type ipv4_addr\; }
<cmdline>:1:1-56: Error: Could not process rule: Numerical result out of range
add set ip crowdsec crowdsec-blackli { type ipv4_addr; }
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
$ sudo nft add set ip crowdsec crowdsec-blackl { type ipv4_addr\; }
$ 

The current PR is limited -- we could have something that truncates and retries, at the cost of code complications and ugly names. We cannot check the actual limits at runtime. Can we flush an object at a time?

[mmetc]

#284