nft: cache: Optimize caching for flush command

nft: cache: Optimize caching for flush command

When flushing all chains and verbose mode is not enabled,
nft_rule_flush() uses a shortcut: It doesn't specify a chain name for
NFT_MSG_DELRULE, so the kernel will flush all existing chains without
user space needing to know which they are.

The above allows to avoid a chain cache, but there's a caveat:
nft_xt_builtin_init() will create base chains as it assumes they are
missing and thereby possibly overrides any non-default chain policies.

Solve this by making nft_xt_builtin_init() cache-aware: If a command
doesn't need a chain cache, there's no need to bother with creating any
non-existing builtin chains, either. For the sake of completeness, also
do nothing if cache is not initialized (although that shouldn't happen).

Signed-off-by: Phil Sutter <phil@nwl.cc>