netfilter: nft_socket: fix erroneous socket assignment

The socket assignment is wrong, see skb_orphan():
When skb->destructor callback is not set, but skb->sk is set, this hits BUG().

Link: https://bugzilla.redhat.com/show_bug.cgi?id=1651813
Fixes: 554ced0a

 ("netfilter: nf_tables: add support for native socket matching")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>