netfilter: nf_tables: do not allow to delete table with owner by handle

nft_table_lookup_byhandle() also needs to validate the netlink PortID
owner when deleting a table by handle.

Fixes: 6001a930

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