tcp: fix more NULL deref after prequeue changes

When I cooked commit c3658e8d0f1 ("tcp: fix possible NULL dereference in
tcp_vX_send_reset()") I missed other spots we could deref a NULL
skb_dst(skb)

Again, if a socket is provided, we do not need skb_dst() to get a
pointer to network namespace : sock_net(sk) is good enough.

[Backport of net-next 0f85feae6b710ced3abad5b2b47d31dfcb956b62]

Bug: 16355602
Change-Id: Ibe1def7979625ee7902bff2f33ec8945b9945948
Reported-by: Dann Frazier <dann.frazier@canonical.com>
Bisected-by: Dann Frazier <dann.frazier@canonical.com>
Tested-by: Dann Frazier <dann.frazier@canonical.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: ca777eff51f7 ("tcp: remove dst refcount false sharing for prequeue mode")
Signed-off-by: David S. Miller <davem@davemloft.net>