bridge: Make br_is_nd_neigh_msg() accept pointer to "const struct sk_buff"

The skb_buff struct in br_is_nd_neigh_msg() is never modified. Mark it as
const.

Signed-off-by: Ted Chen <znscnchen@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250104083846.71612-1-znscnchen@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>