net: Add rx_skb of kfree_skb to raw_tp_null_args[].

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

net: Add rx_skb of kfree_skb to raw_tp_null_args[].

Yan Zhai reported a BPF prog could trigger a null-ptr-deref [0]
in trace_kfree_skb if the prog does not check if rx_sk is NULL.

Commit c53795d48ee8 ("net: add rx_sk to trace_kfree_skb") added
rx_sk to trace_kfree_skb, but rx_sk is optional and could be NULL.

Let's add kfree_skb to raw_tp_null_args[] to let the BPF verifier
validate such a prog and prevent the issue.

Now we fail to load such a prog:

libbpf: prog 'drop': -- BEGIN PROG LOAD LOG --
0: R1=ctx() R10=fp0
; int BPF_PROG(drop, struct sk_buff *skb, void *location, @ kfree_skb_sk_null.bpf.c:21
0: (79) r3 = *(u64 *)(r1 +24)
func 'kfree_skb' arg3 has btf_id 5253 type STRUCT 'sock'
1: R1=ctx() R3_w=trusted_ptr_or_null_sock(id=1)
; bpf_printk("sk: %d, %d\n", sk, sk->__sk_common.skc_family); @ kfree_skb_sk_null.bpf.c:24
1: (69) r4 = *(u16 *)(r3 +16)
R3 invalid mem access 'trusted_ptr_or_null_'
processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0
-- END PROG LOAD LOG --

Note this fix requires commit 838a10bd2ebf ("bpf: Augment raw_tp
arguments with PTR_MAYBE_NULL").

[0]:
BUG: kernel NULL pointer dereference, address: 0000000000000010
PF: supervisor read access in kernel mode
PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
PREEMPT SMP
RIP: 0010:bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
Call Trace:
<TASK>
? __die+0x1f/0x60
? page_fault_oops+0x148/0x420
? search_bpf_extables+0x5b/0x70
? fixup_exception+0x27/0x2c0
? exc_page_fault+0x75/0x170
? asm_exc_page_fault+0x22/0x30
? bpf_prog_5e21a6db8fcff1aa_drop+0x10/0x2d
bpf_trace_run4+0x68/0xd0
? unix_stream_connect+0x1f4/0x6f0
sk_skb_reason_drop+0x90/0x120
unix_stream_connect+0x1f4/0x6f0
__sys_connect+0x7f/0xb0
__x64_sys_connect+0x14/0x20
do_syscall_64+0x47/0xc30
entry_SYSCALL_64_after_hwframe+0x4b/0x53