dsa: mv88e6xxx: Fix MTU definition

stable inclusion
from stable-5.10.71
commit 7b771b12229e82dac66cf0090d0774475a3c662c
bugzilla: 182981 https://gitee.com/openeuler/kernel/issues/I4I3KD

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7b771b12229e82dac66cf0090d0774475a3c662c



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[ Upstream commit b92ce2f5 ]

The MTU passed to the DSA driver is the payload size, typically 1500.
However, the switch uses the frame size when applying restrictions.
Adjust the MTU with the size of the Ethernet header and the frame
checksum. The VLAN header also needs to be included when the frame
size it per port, but not when it is global.

Fixes: 1baf0fac ("net: dsa: mv88e6xxx: Use chip-wide max frame size for MTU")
Reported by: 曹煜 <cao88yu@gmail.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
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: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>