netfilter: nftables: fix possible double hook unregistration with table owner

Skip hook unregistration of owner tables from the netns exit path,
nft_rcv_nl_event() unregisters the table hooks before tearing down
the table content.

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