sit: update dev->needed_headroom in ipip6_tunnel_bind_dev()

sit: update dev->needed_headroom in ipip6_tunnel_bind_dev()

When a tunnel device is bound with the underlying device, its
dev->needed_headroom needs to be updated properly. IPv4 tunnels
already do the same in ip_tunnel_bind_dev(). Otherwise we may
not have enough header room for skb, especially after commit
b17f709a2401 ("gue: TX support for using remote checksum offload option").

Fixes: 32b8a8e59c9c ("sit: add IPv4 over IPv4 support")
Reported-by: Palash Oswal <oswalpalash@gmail.com>
Link: https://lore.kernel.org/netdev/CAGyP=7fDcSPKu6nttbGwt7RXzE3uyYxLjCSE97J64pRxJP8jPA@mail.gmail.com/
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Cong Wang <cong.wang@bytedance.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>