xfrm: Don't accidentally set RTO_ONLINK in decode_session4()

stable inclusion
from stable-v5.10.94
commit 7f2ca96bd26644517ab03357bcf2982cf79c2ff7
bugzilla: https://gitee.com/openeuler/kernel/issues/I531X9

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7f2ca96bd26644517ab03357bcf2982cf79c2ff7



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commit 23e7b1bf upstream.

Similar to commit 94e22389 ("xfrm4: strip ECN bits from tos field"),
clear the ECN bits from iph->tos when setting ->flowi4_tos.
This ensures that the last bit of ->flowi4_tos is cleared, so
ip_route_output_key_hash() isn't going to restrict the scope of the
route lookup.

Use ~INET_ECN_MASK instead of IPTOS_RT_MASK, because we have no reason
to clear the high order bits.

Found by code inspection, compile tested only.

Fixes: 4da3089f ("[IPSEC]: Use TOS when doing tunnel lookups")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>
Acked-by: Xie XiuQi <xiexiuqi@huawei.com>