mptcp: fix for setting remote ipv4mapped address

Commit 1c670b39 ("mptcp: change local addr type of subflow_destroy")
introduced a bug in mptcp_pm_nl_subflow_destroy_doit().

ipv6_addr_set_v4mapped() should be called to set the remote ipv4 address
'addr_r.addr.s_addr' to the remote ipv6 address 'addr_r.addr6', not
'addr_l.addr.addr6', which is the local ipv6 address.

Fixes: 1c670b39 ("mptcp: change local addr type of subflow_destroy")
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250114-net-next-mptcp-fix-remote-addr-v1-1-debcd84ea86f@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>