tcp: Check daddr_cache before use in tracepoint

Running perf in one window to capture tcp_retransmit_skb tracepoint:
    $ perf record -e tcp:tcp_retransmit_skb -a

And causing a retransmission on an active TCP session (e.g., dropping
packets in the receiver, changing MTU on the interface to 500 and back
to 1500) triggers a panic:

[   58.543144] BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[   58.545300] IP: perf_trace_tcp_retransmit_skb+0xd0/0x145
[   58.546770] PGD 0 P4D 0
[   58.547472] Oops: 0000 [#1] SMP
[   58.548328] Modules linked in: vrf
[   58.549262] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.14.0-rc4+ #26
[   58.551004] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.7.5-20140531_083030-gandalf 04/01/2014
[   58.554560] task: ffffffff81a0e540 task.stack: ffffffff81a00000
[   58.555817] RIP: 0010:perf_trace_tcp_retransmit_skb+0xd0/0x145
[   58.557137] RSP: 0018:ffff88003fc03d68 EFLAGS: 00010282
[   58.558292] RAX: 0000000000000000 RBX: ffffe8ffffc0ec80 RCX: ffff880038543098
[   58.559850] RDX: 0400000000000000 RSI: ffff88003fc03d70 RDI: ffff88003fc14b68
[   58.561099] RBP: ffff88003fc03da8 R08: 0000000000000000 R09: ffffea0000d3224a
[   58.562005] R10: ffff88003fc03db8 R11: 0000000000000010 R12: ffff8800385428c0
[   58.562930] R13: ffffe8ffffc0e478 R14: ffffffff81a93a40 R15: ffff88003d4f0c00
[   58.563845] FS:  0000000000000000(0000) GS:ffff88003fc00000(0000) knlGS:0000000000000000
[   58.564873] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   58.565613] CR2: 0000000000000008 CR3: 000000003d68f004 CR4: 00000000000606f0
[   58.566538] Call Trace:
[   58.566865]  <IRQ>
[   58.567140]  __tcp_retransmit_skb+0x4ab/0x4c6
[   58.567704]  ? tcp_set_ca_state+0x22/0x3f
[   58.568231]  tcp_retransmit_skb+0x14/0xa3
[   58.568754]  tcp_retransmit_timer+0x472/0x5e3
[   58.569324]  ? tcp_write_timer_handler+0x1e9/0x1e9
[   58.569946]  tcp_write_timer_handler+0x95/0x1e9
[   58.570548]  tcp_write_timer+0x2a/0x58

Check that daddr_cache is non-NULL before de-referencing.

Fixes: e086101b ("tcp: add a tracepoint for tcp retransmission")
Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>