net: flow_dissector: use DEBUG_NET_WARN_ON_ONCE

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

net: flow_dissector: use DEBUG_NET_WARN_ON_ONCE

The following splat is easy to reproduce upstream as well as in -stable
kernels. Florian Westphal provided the following commit:

d1dab4f71d37 ("net: add and use __skb_get_hash_symmetric_net")

but this complementary fix has been also suggested by Willem de Bruijn
and it can be easily backported to -stable kernel which consists in
using DEBUG_NET_WARN_ON_ONCE instead to silence the following splat
given __skb_get_hash() is used by the nftables tracing infrastructure to
to identify packets in traces.

[69133.561393] ------------[ cut here ]------------
[69133.561404] WARNING: CPU: 0 PID: 43576 at net/core/flow_dissector.c:1104 __skb_flow_dissect+0x134f/
[...]
[69133.561944] CPU: 0 PID: 43576 Comm: socat Not tainted 6.10.0-rc7+ #379
[69133.561959] RIP: 0010:__skb_flow_dissect+0x134f/0x2ad0
[69133.561970] Code: 83 f9 04 0f 84 b3 00 00 00 45 85 c9 0f 84 aa 00 00 00 41 83 f9 02 0f 84 81 fc ff
ff 44 0f b7 b4 24 80 00 00 00 e9 8b f9 ff ff <0f> 0b e9 20 f3 ff ff 41 f6 c6 20 0f 84 e4 ef ff ff 48 8d 7b 12 e8
[69133.561979] RSP: 0018:ffffc90000006fc0 EFLAGS: 00010246
[69133.561988] RAX: 0000000000000000 RBX: ffffffff82f33e20 RCX: ffffffff81ab7e19
[69133.561994] RDX: dffffc0000000000 RSI: ffffc90000007388 RDI: ffff888103a1b418
[69133.562001] RBP: ffffc90000007310 R08: 0000000000000000 R09: 0000000000000000
[69133.562007] R10: ffffc90000007388 R11: ffffffff810cface R12: ffff888103a1b400
[69133.562013] R13: 0000000000000000 R14: ffffffff82f33e2a R15: ffffffff82f33e28
[69133.562020] FS: 00007f40f7131740(0000) GS:ffff888390800000(0000) knlGS:0000000000000000
[69133.562027] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[69133.562033] CR2: 00007f40f7346ee0 CR3: 000000015d200001 CR4: 00000000001706f0
[69133.562040] Call Trace:
[69133.562044] <IRQ>
[69133.562049] ? __warn+0x9f/0x1a0
[ 1211.841384] ? __skb_flow_dissect+0x107e/0x2860
[...]
[ 1211.841496] ? bpf_flow_dissect+0x160/0x160
[ 1211.841753] __skb_get_hash+0x97/0x280
[ 1211.841765] ? __skb_get_hash_symmetric+0x230/0x230
[ 1211.841776] ? mod_find+0xbf/0xe0
[ 1211.841786] ? get_stack_info_noinstr+0x12/0xe0
[ 1211.841798] ? bpf_ksym_find+0x56/0xe0
[ 1211.841807] ? __rcu_read_unlock+0x2a/0x70
[ 1211.841819] nft_trace_init+0x1b9/0x1c0 [nf_tables]
[ 1211.841895] ? nft_trace_notify+0x830/0x830 [nf_tables]
[ 1211.841964] ? get_stack_info+0x2b/0x80
[ 1211.841975] ? nft_do_chain_arp+0x80/0x80 [nf_tables]
[ 1211.842044] nft_do_chain+0x79c/0x850 [nf_tables]