ipv4: Check attribute length for RTA_FLOW in multipath route

stable inclusion
from stable-v5.10.91
commit 914420a2a6c5d72b9e9f0396a988810b6716b4b5
bugzilla: 186187 https://gitee.com/openeuler/kernel/issues/I4SI2C

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=914420a2a6c5d72b9e9f0396a988810b6716b4b5



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commit 664b9c4b upstream.

Make sure RTA_FLOW is at least 4B before using.

Fixes: 4e902c57 ("[IPv4]: FIB configuration using struct fib_config")
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>