esp4: Fix udpencap for local TCP packets.

esp4: Fix udpencap for local TCP packets.

[ Upstream commit 0e78a87306a6f55b1c7bbafad1de62c3975953ca ]

Locally generated TCP packets are usually cloned, so we
do skb_cow_data() on this packets. After that we need to
reload the pointer to the esp header. On udpencap this
header has an offset to skb_transport_header, so take this
offset into account.

This is a backport of:
commit 0e78a87306a ("esp4: Fix udpencap for local TCP packets.")

Fixes: 67d349ed603 ("net/esp4: Fix invalid esph pointer crash")
Fixes: fca11ebde3f0 ("esp4: Reorganize esp_output")
Reported-by: Don Bowman <db@donbowman.ca>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>