nft: Fix for zeroing existent builtin chains

nft: Fix for zeroing existent builtin chains

Previous attempt at fixing for non-existent chains actually broke
functionality by adding a check for NFTNL_CHAIN_HANDLE right after
unsetting the attribute.

The approach was flawed for another reason, too: Base chains added in
the same batch (cf. iptables-restore) have no handle either but zeroing
them may still be sensible.

Instead, make use of the new fake chain annotation which identifies
fakes more reliably.

Fixes: f462975fb8049 ("nft: Fix for zeroing non-existent builtin chains")
Signed-off-by: Phil Sutter <phil@nwl.cc>