selftests/bpf: Workaround for get_stack_rawtp test.

./test_progs-no_alu32 -t get_stack_raw_tp
fails due to:

52: (85) call bpf_get_stack#67
53: (bf) r8 = r0
54: (bf) r1 = r8
55: (67) r1 <<= 32
56: (c7) r1 s>>= 32
; if (usize < 0)
57: (c5) if r1 s< 0x0 goto pc+26
 R0=inv(id=0,smax_value=800) R1_w=inv(id=0,umax_value=800,var_off=(0x0; 0x3ff)) R6=ctx(id=0,off=0,imm=0) R7=map_value(id=0,off=0,ks=4,vs=1600,imm=0) R8_w=inv(id=0,smax_value=800) R9=inv800
; ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
58: (1f) r9 -= r8
; ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
59: (bf) r2 = r7
60: (0f) r2 += r1
regs=1 stack=0 before 52: (85) call bpf_get_stack#67
; ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
61: (bf) r1 = r6
62: (bf) r3 = r9
63: (b7) r4 = 0
64: (85) call bpf_get_stack#67
 R0=inv(id=0,smax_value=800) R1_w=ctx(id=0,off=0,imm=0) R2_w=map_value(id=0,off=0,ks=4,vs=1600,umax_value=800,var_off=(0x0; 0x3ff),s32_max_value=1023,u32_max_value=1023) R3_w=inv(id=0,umax_value=9223372036854776608)
R3 unbounded memory access, use 'var &= const' or 'if (var < const)'

In the C code:
  usize = bpf_get_stack(ctx, raw_data, max_len, BPF_F_USER_STACK);
  if (usize < 0)
          return 0;

  ksize = bpf_get_stack(ctx, raw_data + usize, max_len - usize, 0);
  if (ksize < 0)
          return 0;

We used to have problem with pointer arith in R2.
Now it's a problem with two integers in R3.
'if (usize < 0)' is comparing R1 and makes it [0,800], but R8 stays [-inf,800].
Both registers represent the same 'usize' variable.
Then R9 -= R8 is doing 800 - [-inf, 800]
so the result of "max_len - usize" looks unbounded to the verifier while
it's obvious in C code that "max_len - usize" should be [0, 800].

To workaround the problem convert ksize and usize variables from int to long.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/test_get_stack_rawtp.c

@@ -57,8 +57,9 @@ struct {
 SEC("raw_tracepoint/sys_enter")
 int bpf_prog1(void *ctx)
 {
-	int max_len, max_buildid_len, usize, ksize, total_size;
+	int max_len, max_buildid_len, total_size;
 	struct stack_trace_t *data;
+	long usize, ksize;
 	void *raw_data;
 	__u32 key = 0;