LoongArch: BPF: Rename and refactor validate_code()

1. Rename the existing validate_code() to validate_ctx()
2. Factor out the code validation handling into a new helper
   validate_code()

Then:

* validate_code() is used to check the validity of code.
* validate_ctx() is used to check both code validity and table entry
  correctness.

The new validate_code() will be used in subsequent changes.

Reviewed-by: Hengqi Chen <hengqi.chen@gmail.com>
Co-developed-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
Signed-off-by: Chenghao Duan <duanchenghao@kylinos.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>