vxlan: Pull inner IP header in vxlan_xmit_one().

Ensure the inner IP header is part of the skb's linear data before
setting old_iph. Otherwise, on a non-linear skb, old_iph could point
outside of the packet data.

Unlike classical VXLAN, which always encapsulates Ethernet packets,
VXLAN-GPE can transport IP packets directly. In that case, we need to
look at skb->protocol to figure out if an Ethernet header is present.

Fixes: d342894c

 ("vxlan: virtual extensible lan")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Link: https://patch.msgid.link/2aa75f6fa62ac9dbe4f16ad5ba75dd04a51d4b99.1718804000.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>