nft: Fix for zeroing non-existent builtin chains

nft: Fix for zeroing non-existent builtin chains

Trying to zero a specific rule in an entirely empty ruleset caused an
error:

| # nft flush ruleset
| # iptables-nft -Z INPUT
| iptables v1.8.10 (nf_tables):  CHAIN_ZERO failed (No such file or directory): chain INPUT

To fix this, start by faking any non-existing builtin chains so verbose
mode prints all the would-be-flushed chains. Later set 'skip' flag if
given chain is a fake one (indicated by missing HANDLE attribute).
Finally cover for concurrent ruleset updates by checking whether the
chain exists.

This bug seems to exist for a long time already, Fixes tag identified
via git-bisect. This patch won't apply to such old trees though, but
calling nft_xt_builtin_init() from nft_chain_zero_counters() should work
there.

Fixes: a6ce0c65d3a39 ("xtables: Optimize nft_chain_zero_counters()")
Signed-off-by: Phil Sutter <phil@nwl.cc>