mlx5: fix get_ip_proto()

mainline inclusion
from mainline-4.20
commit ef6fcd45
category: bugfix
bugzilla: 6077
CVE: NA

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IP header is not necessarily located right after struct ethhdr,
there could be multiple 802.1Q headers in between, this is why
we call __vlan_get_protocol().

Fixes: fe1dc069 ("net/mlx5e: don't set CHECKSUM_COMPLETE on SCTP packets")
Cc: Alaa Hleihel <alaa@mellanox.com>
Cc: Or Gerlitz <ogerlitz@mellanox.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Keefe LIU <liuqifa@huawei.com>
Reviewed-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>