strncmp: invalid BPF_LD_IMM insn

I met an issue of invalid BPF_LD_IMM insn when use strncmp(key.comm, "xxx", sizeof(key.comm));
After I rewrite the strncmp function with C language, it works well, but I feel it is inconvenient, so I want ask whether you have good idea, thank you.

```
static int du_strncmp(const char *cs, const char *ct, size_t count)
{
unsigned char c1, c2;

while (count) {
c1 = *cs++;
c2 = *ct++;
if (c1 != c2)
return c1 < c2 ? -1 : 1;
if (!c1)
break;
count--;
}
return 0;
}

int print_submit_bio(struct pt_regs *ctx, struct bio *bio)
{
struct key_t key = {};
bpf_get_current_comm(&key.comm, sizeof(key.comm));
//du_strncmp(key.comm, "xxx", sizeof(xxx));
strncmp(key.comm, "xxx", sizeof(xxx));
....
```

Reproduce the print_submit_bio example using strncmp(key.comm, "xxx", sizeof(xxx)) and compare it with the provided C implementation. Inspect the generated BPF around the reported BPF_LD_IMM instruction; done means the strncmp form loads as valid BPF and works without requiring the custom replacement.

Written by the indexing model from the issue text.