fix a verifier failure

when running with latest linus tree and net-next, the python test
tests/python/test_tracepoint.py failed with the following
symptoms:
```
......
 R0=map_value(id=0,off=0,ks=4,vs=64,imm=0) R6=map_value(id=0,off=0,ks=4,vs=64,imm=0) R7=ctx(id=0,off=0,imm=0) R10=fp0,call_-1
34: (69) r1 = *(u16 *)(r7 +8)
35: (67) r1 <<= 48
36: (c7) r1 s>>= 48
37: (0f) r7 += r1
math between ctx pointer and register with unbounded min value is not allowed
......
```

The reason of failure is due to added tightening in verifier introduced by
the following commit:
```
commit f4be569a429987343e3f424d3854b3622ffae436
Author: Alexei Starovoitov <[email protected]>
Date: Mon Dec 18 20:12:00 2017 -0800

bpf: fix integer overflows
......

```

The patch changes `offset` type in `ctx + offset` from signed to
unsigned so that we do not have `unbounded min value` so the
test trace_tracepoint.py can pass.

Signed-off-by: Yonghong Song <[email protected]>

src/cc/export/helpers.h

@@ -598,14 +598,14 @@ int tracepoint__##category##__##event(struct tracepoint__##category##__##event *
 
 #define TP_DATA_LOC_READ_CONST(dst, field, length) \
  do { \
- short __offset = args->data_loc_##field & 0xFFFF;  \
+ unsigned short __offset = args->data_loc_##field & 0xFFFF; \
  bpf_probe_read((void *)dst, length, (char *)args + __offset); \
  } while (0);
 
 #define TP_DATA_LOC_READ(dst, field) \
  do { \
- short __offset = args->data_loc_##field & 0xFFFF;  \
- short __length = args->data_loc_##field >> 16;  \
+ unsigned short __offset = args->data_loc_##field & 0xFFFF; \
+ unsigned short __length = args->data_loc_##field >> 16; \
  bpf_probe_read((void *)dst, __length, (char *)args + __offset); \
  } while (0);