tcp: ipv6: free ireq->ipv6_opt after an RCU grace period

ireq->ipv6_opt is only set by calipso_req_setattr(), when a CALIPSO
NetLabel mapping is configured.

Request socket migration (net.ipv4.tcp_migrate_req=1, or a
BPF_SK_REUSEPORT_SELECT_OR_MIGRATE program) clones the request
socket with inet_reqsk_clone(), so that the old request socket and
its clone share the same ipv6_opt, then reqsk_migrate_reset() clears
the pointer in the old one.

Another cpu holding a reference on the old request socket can still
be using these options, sending a SYNACK from tcp_v6_send_synack()
or creating a child in tcp_v6_syn_recv_sock(), when the clone
is freed. tcp_v6_reqsk_destructor() frees ipv6_opt with a plain
kfree(), leading to a use-after-free.

Both readers already run under RCU, like their IPv4 counterparts.
Use kfree_rcu() in tcp_v6_reqsk_destructor(), as commit e3f33b0a1d89
("ipv4: free inet_opt and ireq_opt after an RCU grace period") did
for ireq_opt.

Fixes: c905dee62232 ("tcp: Migrate TCP_NEW_SYN_RECV requests at retransmitting SYN+ACKs.")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/netdev/179369559062.488856.15441115725436277776@kernel.org/
Signed-off-by: Eric Dumazet <edumazet@kernel.org>
Reviewed-by: Hangbin Liu <liuhangbin@kylinos.cn>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Tested-by: Linkui Xiao <xiaolinkui@kylinos.cn>
Link: https://patch.msgid.link/20261103090011.4128420-1-edumazet@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>