netfilter: nf_tables: fix flush after rule deletion in the same batch

Flush after rule deletion bogusly hits -ENOENT. Skip rules that have
been already from nft_delrule_by_chain() which is always called from the
flush path.

Fixes: cf9dc09d ("netfilter: nf_tables: fix missing rules flushing per table")
Reported-by: Phil Sutter <phil@nwl.cc>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>