appletalk: Fix skb allocation size in loopback case

stable inclusion
from stable-5.10.28
commit c805f215e9c587db246c20e985dda5a768bab3e7
bugzilla: 51779

--------------------------------

[ Upstream commit 39935dcc ]

If a DDP broadcast packet is sent out to a non-gateway target, it is
also looped back. There is a potential for the loopback device to have a
longer hardware header length than the original target route's device,
which can result in the skb not being created with enough room for the
loopback device's hardware header. This patch fixes the issue by
determining that a loopback will be necessary prior to allocating the
skb, and if so, ensuring the skb has enough room.

This was discovered while testing a new driver that creates a LocalTalk
network interface (LTALK_HLEN = 1). It caused an skb_under_panic.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by:   Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>