nft: cache: Fix iptables-save segfault under stress

nft: cache: Fix iptables-save segfault under stress

If kernel ruleset is constantly changing, code called by
nft_is_table_compatible() may crash: For each item in table's chain
list, nft_is_chain_compatible() is called. This in turn calls
nft_build_cache() to fetch chain's rules. Though if kernel genid has changed
meanwhile, cache is flushed and rebuilt from scratch, thereby freeing
table's chain list - the foreach loop in nft_is_table_compatible() then
operates on freed memory.

A simple reproducer (may need a few calls):

| RULESET='*filter
| :INPUT ACCEPT [10517:1483527]
| :FORWARD ACCEPT [0:0]
| :OUTPUT ACCEPT [1714:105671]
| COMMIT
| '
|
| for ((i = 0; i < 100; i++)); do
|         iptables-nft-restore <<< "$RULESET" &
| done &
| iptables-nft-save

To fix the problem, basically revert commit ab1cd3b510fa5 ("nft: ensure
cache consistency") so that __nft_build_cache() no longer flushes the
cache. Instead just record kernel's genid when fetching for the first
time. If kernel rule set changes until the changes are committed, the
commit simply fails and local cache is being rebuilt.

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