netfilter: nf_tables: prepare nft audit for set element compaction

netfilter: nf_tables: prepare nft audit for set element compaction

nftables audit log format emits the number of added/deleted rules, sets,
set elements and so on, to userspace:

    table=t1 family=2 entries=4 op=nft_register_set
                      ~~~~~~~~~

At this time, the 'entries' key is the number of transactions that will
be applied.

The upcoming set element compression will coalesce subsequent
adds/deletes to the same set requests in the same transaction
request to conseve memory.

Without this patch, we'd under-report the number of altered elements.

Increment the audit counter by the number of elements to keep the reported
entries value the same.

Without this, nft_audit.sh selftest fails because the recorded
(expected) entries key is smaller than the expected one.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>