xtables-save: Ignore uninteresting tables

xtables-save: Ignore uninteresting tables

When running iptables-nft-save with other tables present, the dump
succeeded but the tool complained about those other tables. In an
environment where iptables-nft and nftables are uses in parallel, this
is an expected situation, so only complain about incompatible builtin
tables.

While being at it, move the table existence check from __do_output()
into do_output() since the former may be called from
nft_for_each_table() in which case the table is guaranteed to exist.

Also use nft_table_builtin_find() in nft_is_table_compatible() instead
of open-coding the search by name in h->tables.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>