xfrm: Clear low order bits of ->flowi4_tos in decode_session4().

Commit 23e7b1b ("xfrm: Don't accidentally set RTO_ONLINK in
decode_session4()") fixed a problem where decode_session4() could
erroneously set the RTO_ONLINK flag for IPv4 route lookups. This
problem was reintroduced when decode_session4() was modified to
use the flow dissector.

Fix this by clearing again the two low order bits of ->flowi4_tos.
Found by code inspection, compile tested only.

Fixes: 7a02070 ("xfrm: policy: replace session decode with flow dissector")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: Guillaume Nault

net/xfrm/xfrm_policy.c

@@ -3416,7 +3416,7 @@ decode_session4(const struct xfrm_flow_keys *flkeys, struct flowi *fl, bool reve
 	}
 
 	fl4->flowi4_proto = flkeys->basic.ip_proto;
-	fl4->flowi4_tos = flkeys->ip.tos;
+	fl4->flowi4_tos = flkeys->ip.tos & ~INET_ECN_MASK;
 }
 
 #if IS_ENABLED(CONFIG_IPV6)