batman-adv: dat: ensure accessible eth_hdr proto field

In the Linux kernel, the following vulnerability has been resolved: batman-adv: dat: ensure accessible eth_hdr proto field When batadv_get_vid() accesses the proto field of the ethernet header, it is not checking if the data itself is accessible. The caller is responsible for it. But in contrast to other call sites, batadv_dat_get_vid() and its caller didn't make sure this is true. This could have caused an out-of-bounds access.