ipv4: check for NULL idev in ip_route_use_hint()

In the Linux kernel, the following vulnerability has been resolved:

ipv4: check for NULL idev in ip_route_use_hint()

syzbot was able to trigger a NULL deref in fib_validate_source()
in an old tree [1].

It appears the bug exists in latest trees.

All calls to __in_dev_get_rcu() must be checked for a NULL result.

[1]
general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 2 PID: 3257 Comm: syz-executor.3 Not tainted 5.10.0-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:fib_validate_source+0xbf/0x15a0 net/ipv4/fib_frontend.c:425
Code: 18 f2 f2 f2 f2 42 c7 44 20 23 f3 f3 f3 f3 48 89 44 24 78 42 c6 44 20 27 f3 e8 5d 88 48 fc 4c 89 e8 48 c1 e8 03 48 89 44 24 18 <42> 80 3c 20 00 74 08 4c 89 ef e8 d2 15 98 fc 48 89 5c 24 10 41 bf
RSP: 0018:ffffc900015fee40 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88800f7a4000 RCX: ffff88800f4f90c0
RDX: 0000000000000000 RSI: 0000000004001eac RDI: ffff8880160c64c0
RBP: ffffc900015ff060 R08: 0000000000000000 R09: ffff88800f7a4000
R10: 0000000000000002 R11: ffff88800f4f90c0 R12: dffffc0000000000
R13: 0000000000000000 R14: 0000000000000000 R15: ffff88800f7a4000
FS: 00007f938acfe6c0(0000) GS:ffff888058c00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f938acddd58 CR3: 000000001248e000 CR4: 0000000000352ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
ip_route_use_hint+0x410/0x9b0 net/ipv4/route.c:2231
ip_rcv_finish_core+0x2c4/0x1a30 net/ipv4/ip_input.c:327
ip_list_rcv_finish net/ipv4/ip_input.c:612 [inline]
ip_sublist_rcv+0x3ed/0xe50 net/ipv4/ip_input.c:638
ip_list_rcv+0x422/0x470 net/ipv4/ip_input.c:673
__netif_receive_skb_list_ptype net/core/dev.c:5572 [inline]
__netif_receive_skb_list_core+0x6b1/0x890 net/core/dev.c:5620
__netif_receive_skb_list net/core/dev.c:5672 [inline]
netif_receive_skb_list_internal+0x9f9/0xdc0 net/core/dev.c:5764
netif_receive_skb_list+0x55/0x3e0 net/core/dev.c:5816
xdp_recv_frames net/bpf/test_run.c:257 [inline]
xdp_test_run_batch net/bpf/test_run.c:335 [inline]
bpf_test_run_xdp_live+0x1818/0x1d00 net/bpf/test_run.c:363
bpf_prog_test_run_xdp+0x81f/0x1170 net/bpf/test_run.c:1376
bpf_prog_test_run+0x349/0x3c0 kernel/bpf/syscall.c:3736
__sys_bpf+0x45c/0x710 kernel/bpf/syscall.c:5115
__do_sys_bpf kernel/bpf/syscall.c:5201 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5199 [inline]
__x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5199