udp: better wmem accounting on gso

skb_segment by default transfers allocated wmem from the gso skb
to the tail of the segment list. This underreports real truesize
of the list, especially if the tail might be dropped.

Similar to tcp_gso_segment, update wmem_alloc with the aggregate
list truesize and make each segment responsible for its own
share by setting skb->destructor.

Clear gso_skb->destructor prior to calling skb_segment to skip
the default assignment to tail.

Change-Id: I8f36b6d134f35fa9e4cbb012fcc9a3efcc91ba55
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: ad405857b174ed31a97982bb129c320d03321cf5
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next.git


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