ipv6: record frag_max_size in atomic fragments in input path

stable inclusion
from linux-4.19.193
commit ea9ef822d541859b65696b62647ccc4ab43c1129

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

[ Upstream commit e29f011e ]

Commit dbd1759e ("ipv6: on reassembly, record frag_max_size")
filled the frag_max_size field in IP6CB in the input path.
The field should also be filled in case of atomic fragments.

Fixes: dbd1759e ('ipv6: on reassembly, record frag_max_size')
Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>