netfilter: nf_tables: allow to fetch set elements when table has an owner

NFT_MSG_GETSETELEM returns -EPERM when fetching set elements that belong
to table that has an owner. This results in empty set/map listing from
userspace.

Fixes: 6001a930 ("netfilter: nftables: introduce table ownership")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>