Put callx register into dst (#202)

* Clang puts callx register into imm not src_reg

Fix bug pointed out by Will.
Still waiting to confirm what gcc does.

Signed-off-by: Dave Thaler <[email protected]>

* Put callx register into dst_reg not imm or src_reg

Per latest mailing list discussion at
https://mailarchive.ietf.org/arch/msg/bpf/Vx1H3ViPUWoGKNssCO22lOIjyXU/

Signed-off-by: Dave Thaler <[email protected]>

---------

Signed-off-by: Dave Thaler <[email protected]>
Co-authored-by: Alan Jowett <[email protected]>

Author: dthaler

src/bpf_assembler.cc

@@ -311,7 +311,7 @@ typedef class _bpf_assembler
             if (mode == "helper") {
                 if (target.starts_with('%')) {
                     inst.opcode |= EBPF_SRC_REG;
-                    inst.imm = _decode_register(target);
+                    inst.dst = _decode_register(target);
                 } else {
                     inst.opcode |= EBPF_SRC_IMM;
                     inst.imm = _decode_imm32(target);