nft: Simplify flush_chain_cache()

nft: Simplify flush_chain_cache()

With all the checks for 'tablename' being non-NULL, this code was rather
stupid and really hard to read. And the fix is indeed quite simple: If a
table name was given, use nft_table_builtin_find() and just flush its
chain cache. Otherwise iterate over all builtin tables without any
conditionals for 'tablename'.

Fixes: d4b0d248cc057 ("nft: Reduce indenting level in flush_chain_cache()")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>