vxlan: use neigh_ha_snapshot() in route_shortcircuit()

In the Linux kernel, the following vulnerability has been resolved: vxlan: use neigh_ha_snapshot() in route_shortcircuit() The neighbour hardware address n->ha can be updated asynchronously by the neighbour subsystem, protected by n->ha_lock seqlock. Reading n->ha without holding the seqlock loop can lead to torn reads or reading a partially updated MAC address. Use neigh_ha_snapshot() in route_shortcircuit() to safely copy n->ha under read_seqbegin()/read_seqretry() lock protection before using it. Note that arp_reduce() and neigh_reduce() seem to have the same issue left for future patches.