bpf_validate() uses BPF_RVAL() when it should use BPF_SRC()

[Guy Harris]

In bpf_validate, when it checks whether the divisor in a BPF_DIV instruction
is a constant 0, it does

                        case BPF_DIV:
                                /*
                                 * Check for constant division by 0.
                                 */
                                if (BPF_RVAL(p->code) == BPF_K && p->k == 0)
                                        return 0;
                                break;

BPF_RVAL() is the macro to get the return value of a RET instruction; it
extracts the 0x18 bits.  The BPF_DIV opcode is 0x30, which has the 0x10 bit
set; a BPF_DIV instruction with a constant 0 as the divisor would be
BPF_DIV|BPF_K, which is 0x30; BPF_RVAL(p->code) would be 0x10, which isn't
equal to BPF_K, which is 0x00.

The macro to get the source argument of an arithmetic instruction is
BPF_SRC(), which extracts only the 0x08 bit; BPF_SRC(p->code) would be 0x00,
which is equal to BPF_K, so it should be doing

                        case BPF_DIV:
                                /*
                                 * Check for constant division by 0.
                                 */
                                if (BPF_SRC(p->code) == BPF_K && p->k == 0)
                                        return 0;
                                break;