netfilter: nf_tables: skip netlink portID validation if zero

nft_table_lookup() allows us to obtain the table object by the name and
the family. The netlink portID validation needs to be skipped for the
dump path, since the ownership only applies to commands to update the
given table. Skip validation if the specified netlink PortID is zero
when calling nft_table_lookup().

Fixes: 6001a930

 ("netfilter: nftables: introduce table ownership")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>