bpf: free up BPF_JMP | BPF_CALL | BPF_X opcode

free up BPF_JMP | BPF_CALL | BPF_X opcode to be used by actual
indirect call by register and use kernel internal opcode to
mark call instruction into bpf_tail_call() helper.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

kernel/bpf/verifier.c

@@ -3469,7 +3469,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			 * that doesn't support bpf_tail_call yet
 			 */
 			insn->imm = 0;
-			insn->code |= BPF_X;
+			insn->code = BPF_JMP | BPF_TAIL_CALL;
 			continue;
 		}