nft: cache: Fix nft_release_cache() under stress

nft: cache: Fix nft_release_cache() under stress

iptables-nft-restore calls nft_action(h, NFT_COMPAT_COMMIT) for each
COMMIT line in input. When restoring a dump containing multiple large
tables, chances are nft_rebuild_cache() has to run multiple times.

If the above happens, consecutive table contents are added to __cache[1]
which nft_rebuild_cache() then frees, so next commit attempt accesses
invalid memory.

Fix this by making nft_release_cache() (called after each successful
commit) return things into pre-rebuild state again, but keeping the
fresh cache copy.

Fixes: f6ad231d698c7 ("nft: keep original cache in case of ERESTART")
Signed-off-by: Phil Sutter <phil@nwl.cc>