mptcp: map v4 address to v6 when destroying subflow

Address family of server side mismatches with that of client side, like
in "userspace pm add & remove address" test:

    userspace_pm_add_addr $ns1 10.0.2.1 10
    userspace_pm_rm_sf $ns1 "::ffff:10.0.2.1" $SUB_ESTABLISHED

That's because on the server side, the family is set to AF_INET6 and the
v4 address is mapped in a v6 one.

This patch fixes this issue. In mptcp_pm_nl_subflow_destroy_doit(), before
checking local address family with remote address family, map an IPv4
address to an IPv6 address if the pair is a v4-mapped address.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/387
Fixes: 702c2f64 ("mptcp: netlink: allow userspace-driven subflow establishment")
Cc: stable@vger.kernel.org
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240223-upstream-net-20240223-misc-fixes-v1-1-162e87e48497@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/pm_userspace.c

@@ -495,6 +495,16 @@ int mptcp_pm_nl_subflow_destroy_doit(struct sk_buff *skb, struct genl_info *info
 		goto destroy_err;
 	}
 
+#if IS_ENABLED(CONFIG_MPTCP_IPV6)
+	if (addr_l.family == AF_INET && ipv6_addr_v4mapped(&addr_r.addr6)) {
+		ipv6_addr_set_v4mapped(addr_l.addr.s_addr, &addr_l.addr6);
+		addr_l.family = AF_INET6;
+	}
+	if (addr_r.family == AF_INET && ipv6_addr_v4mapped(&addr_l.addr6)) {
+		ipv6_addr_set_v4mapped(addr_r.addr.s_addr, &addr_r.addr6);
+		addr_r.family = AF_INET6;
+	}
+#endif
 	if (addr_l.family != addr_r.family) {
 		GENL_SET_ERR_MSG(info, "address families do not match");
 		err = -EINVAL;