bareudp: Pull inner IP header on xmit.

Both bareudp_xmit_skb() and bareudp6_xmit_skb() read their skb's inner
IP header to get its ECN value (with ip_tunnel_ecn_encap()). Therefore
we need to ensure that the inner IP header is part of the skb's linear
data.

Fixes: 571912c6 ("net: UDP tunnel encapsulation module for tunnelling different protocols like MPLS, IP, NSH etc.")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/267328222f0a11519c6de04c640a4f87a38ea9ed.1726046181.git.gnault@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>