netfilter: nft_dynset: continue to next expr if _OP_ADD succeeded

netfilter: nft_dynset: continue to next expr if _OP_ADD succeeded

Currently, after adding the following nft rules:
  # nft add set x target1 { type ipv4_addr \; flags timeout \;}
  # nft add rule x y set add ip daddr timeout 1d @target1 counter

the counters will always be zero despite of the elements are added
to the dynamic set "target1" or not, as we will break the nft expr
traversal unconditionally:
  # nft list ruleset
  ...
  set target1 {
      ...
      elements = { 8.8.8.8 expires 23h59m53s}
  }
  chain output {
      ...
      set add ip daddr timeout 1d @target1 counter packets 0 bytes 0
                                                           ^       ^
      ...
  }

Since we add the elements to the set successfully, we should continue
to the next expression.

Additionally, if elements are added to "flow table" successfully, we
will _always_ continue to the next expr, even if the operation is
_OP_ADD. So it's better to keep them to be consistent.

Fixes: 22fe54d5fefc ("netfilter: nf_tables: add support for dynamic set updates")
Reported-by: Robert White <rwhite@pobox.com>
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>