ipv6: fix potential crash in ip6_datagram_dst_update()

Willem forgot to change one of the calls to fl6_sock_lookup(),
which can now return an error or NULL.

syzbot reported :

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 31763 Comm: syz-executor.0 Not tainted 5.2.0-rc6+ #63
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:ip6_datagram_dst_update+0x559/0xc30 net/ipv6/datagram.c:83
Code: 00 00 e8 ea 29 3f fb 4d 85 f6 0f 84 96 04 00 00 e8 dc 29 3f fb 49 8d 7e 20 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 16 06 00 00 4d 8b 6e 20 e8 b4 29 3f fb 4c 89 ee
RSP: 0018:ffff88809ba97ae0 EFLAGS: 00010207
RAX: dffffc0000000000 RBX: ffff8880a81254b0 RCX: ffffc90008118000
RDX: 0000000000000003 RSI: ffffffff86319a84 RDI: 000000000000001e
RBP: ffff88809ba97c10 R08: ffff888065e9e700 R09: ffffed1015d26c80
R10: ffffed1015d26c7f R11: ffff8880ae9363fb R12: ffff8880a8124f40
R13: 0000000000000001 R14: fffffffffffffffe R15: ffff88809ba97b40
FS:  00007f38e606a700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000202c0140 CR3: 00000000a026a000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __ip6_datagram_connect+0x5e9/0x1390 net/ipv6/datagram.c:246
 ip6_datagram_connect+0x30/0x50 net/ipv6/datagram.c:269
 ip6_datagram_connect_v6_only+0x69/0x90 net/ipv6/datagram.c:281
 inet_dgram_connect+0x14a/0x2d0 net/ipv4/af_inet.c:571
 __sys_connect+0x264/0x330 net/socket.c:1824
 __do_sys_connect net/socket.c:1835 [inline]
 __se_sys_connect net/socket.c:1832 [inline]
 __x64_sys_connect+0x73/0xb0 net/socket.c:1832
 do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4597c9
Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f38e6069c78 EFLAGS: 00000246 ORIG_RAX: 000000000000002a
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 00000000004597c9
RDX: 000000000000001c RSI: 0000000020000040 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f38e606a6d4
R13: 00000000004bfd07 R14: 00000000004d1838 R15: 00000000ffffffff
Modules linked in:
RIP: 0010:ip6_datagram_dst_update+0x559/0xc30 net/ipv6/datagram.c:83
Code: 00 00 e8 ea 29 3f fb 4d 85 f6 0f 84 96 04 00 00 e8 dc 29 3f fb 49 8d 7e 20 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <80> 3c 02 00 0f 85 16 06 00 00 4d 8b 6e 20 e8 b4 29 3f fb 4c 89 ee

Fixes: 59c820b2 ("ipv6: elide flowlabel check if no exclusive leases exist")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>