udp: Do not copy destructor if one is not present

This patch makes it so that if a destructor is not present we avoid trying
to update the skb socket or any reference counting that would be associated
with the NULL socket and/or descriptor. By doing this we can support
traffic coming from another namespace without any issues.

Change-Id: I61155db99242c0c39f0a19f7a455c0a90d1ac8f3
CRs-Fixed: 2351793
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 04d55b257cbb9eb6c722410251d3b7df00835c6c
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git


Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>