batman-adv: dat: ensure accessible eth_hdr proto field

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.

Cc: stable@vger.kernel.org
Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: be1db4f6615b ("batman-adv: make the Distributed ARP Table vlan aware")
Signed-off-by: Sven Eckelmann <sven@narfation.org>