arm64: csum: Fix handling of bad packets

stable inclusion
from linux-4.19.137
commit 0be9b57b5bb5a1e643624ea68decc4a8a14ffda6

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

[ Upstream commit 05fb3dbd ]

Although iph is expected to point to at least 20 bytes of valid memory,
ihl may be bogus, for example on reception of a corrupt packet. If it
happens to be less than 5, we really don't want to run away and
dereference 16GB worth of memory until it wraps back to exactly zero...

Fixes: 0e455d8e ("arm64: Implement optimised IP checksum helpers")
Reported-by: guodeqing <geffrey.guo@huawei.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>