CVE-2026-74473: vxlan: use pskb_network_may_pull() in route_shortcircuit()

In the Linux kernel, the following vulnerability has been resolved:

vxlan: use pskbnetworkmaypull() in routeshortcircuit()

routeshortcircuit() currently calls pskbmaypull(skb, sizeof(struct iphdr)) (or ipv6hdr), which checks if bytes are available starting from skb->data.

However, in vxlanxmit(), skb->data points to the MAC header, so skbnetworkoffset(skb) is ETHHLEN (14 bytes). Using pskbmaypull(skb, 20) only checks 20 bytes from skb->data (which is 14 bytes MAC header + 6 bytes of IP header), leaving the rest of the IP header potentially un-pulled in non-linear frags. Subsequent dereferences of iphdr(skb)->daddr can read beyond the pulled linear buffer length.

Fix this by using pskbnetworkmaypull(), which adds skbnetworkoffset(skb) to the length check to ensure the full network header is present in the linear buffer.