xfrm: Fix transport mode skb control buffer usage.

A recent commit introduced a new struct xfrm_trans_cb
that is used with the sk_buff control buffer. Unfortunately
it placed the structure in front of the control buffer and
overlooked that the IPv4/IPv6 control buffer is still needed
for some layer 4 protocols. As a result the IPv4/IPv6 control
buffer is overwritten with this structure. Fix this by setting
a apropriate header in front of the structure.

Fixes acf568ee ("xfrm: Reinject transport-mode packets ...")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>