net: check and errout if res->fi is NULL when RTM_F_FIB_MATCH is set

Syzkaller hit 'general protection fault in fib_dump_info' bug on
commit 4.13-rc5..

Guilty file: net/ipv4/fib_semantics.c

kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Modules linked in:
CPU: 0 PID: 2808 Comm: syz-executor0 Not tainted 4.13.0-rc5 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
Ubuntu-1.8.2-1ubuntu1 04/01/2014
task: ffff880078562700 task.stack: ffff880078110000
RIP: 0010:fib_dump_info+0x388/0x1170 net/ipv4/fib_semantics.c:1314
RSP: 0018:ffff880078117010 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 00000000000000fe RCX: 0000000000000002
RDX: 0000000000000006 RSI: ffff880078117084 RDI: 0000000000000030
RBP: ffff880078117268 R08: 000000000000000c R09: ffff8800780d80c8
R10: 0000000058d629b4 R11: 0000000067fce681 R12: 0000000000000000
R13: ffff8800784bd540 R14: ffff8800780d80b5 R15: ffff8800780d80a4
FS:  00000000022fa940(0000) GS:ffff88007fc00000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004387d0 CR3: 0000000079135000 CR4: 00000000000006f0
Call Trace:
  inet_rtm_getroute+0xc89/0x1f50 net/ipv4/route.c:2766
  rtnetlink_rcv_msg+0x288/0x680 net/core/rtnetlink.c:4217
  netlink_rcv_skb+0x340/0x470 net/netlink/af_netlink.c:2397
  rtnetlink_rcv+0x28/0x30 net/core/rtnetlink.c:4223
  netlink_unicast_kernel net/netlink/af_netlink.c:1265 [inline]
  netlink_unicast+0x4c4/0x6e0 net/netlink/af_netlink.c:1291
  netlink_sendmsg+0x8c4/0xca0 net/netlink/af_netlink.c:1854
  sock_sendmsg_nosec net/socket.c:633 [inline]
  sock_sendmsg+0xca/0x110 net/socket.c:643
  ___sys_sendmsg+0x779/0x8d0 net/socket.c:2035
  __sys_sendmsg+0xd1/0x170 net/socket.c:2069
  SYSC_sendmsg net/socket.c:2080 [inline]
  SyS_sendmsg+0x2d/0x50 net/socket.c:2076
  entry_SYSCALL_64_fastpath+0x1a/0xa5
  RIP: 0033:0x4512e9
  RSP: 002b:00007ffc75584cc8 EFLAGS: 00000216 ORIG_RAX:
  000000000000002e
  RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00000000004512e9
  RDX: 0000000000000000 RSI: 0000000020f2cfc8 RDI: 0000000000000003
  RBP: 000000000000000e R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000216 R12: fffffffffffffffe
  R13: 0000000000718000 R14: 0000000020c44ff0 R15: 0000000000000000
  Code: 00 0f b6 8d ec fd ff ff 48 8b 85 f0 fd ff ff 88 48 17 48 8b 45
  28 48 8d 78 30 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03
  <0f>
  b6 04 02 84 c0 74 08 3c 03 0f 8e cb 0c 00 00 48 8b 45 28 44
  RIP: fib_dump_info+0x388/0x1170 net/ipv4/fib_semantics.c:1314 RSP:
  ffff880078117010
---[ end trace 254a7af28348f88b ]---

This patch adds a res->fi NULL check.

example run:
$ip route get 0.0.0.0 iif virt1-0
broadcast 0.0.0.0 dev lo
    cache <local,brd> iif virt1-0

$ip route get 0.0.0.0 iif virt1-0 fibmatch
RTNETLINK answers: No route to host

Reported-by: idaifish <idaifish@gmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Fixes: b61798130f1b ("net: ipv4: RTM_GETROUTE: return matched fib result when requested")
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
