BPF_FUNC_get_prandom_u32 limitation

[QiongwenXu]

we use the following ways to deal with a program with random helper functions:

1. all instructions CALL BPF_FUNC_get_prandom_u32 in the original program won't be modified
2. new instruction CALL BPF_FUNC_get_prandom_u32 won't be generated
3. interpreter: get random value from pseudo random values in the input
4. validator: BPF_FUNC_get_prandom_u32 instructions with the same instruction id in two programs are set as the same symbolic value