udp6: Fix race condition in udp6_sendmsg & connect

Syzkaller got the following report:
BUG: KASAN: use-after-free in sk_setup_caps+0x621/0x690 net/core/sock.c:2018
Read of size 8 at addr ffff888027f82780 by task syz-executor276/3255

The function sk_setup_caps (called by ip6_sk_dst_store_flow->
ip6_dst_store) referenced already freed memory as this memory was
freed by parallel task in udpv6_sendmsg->ip6_sk_dst_lookup_flow->
sk_dst_check.

          task1 (connect)              task2 (udp6_sendmsg)
        sk_setup_caps->sk_dst_set |
                                  |  sk_dst_check->
                                  |      sk_dst_set
                                  |      dst_release
        sk_setup_caps references  |
        to already freed dst_entry|

The reason for this race condition is: sk_setup_caps() keeps using
the dst after transferring the ownership to the dst cache.

Found by Linux Verification Center (linuxtesting.org) with syzkaller.

Fixes: 1da177e4

 ("Linux-2.6.12-rc2")
Signed-off-by: Vladislav Efanov <VEfanov@ispras.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>