mttcg: Add missing tb_lock/unlock() in cpu_exec_step()

The recent patch enabling lock assertions uncovered the missing lock
acquisition in cpu_exec_step(). This patch adds them.
Signed-off-by: NPranith Kumar <bobby.prani@gmail.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <rth@twiddle.net>

cpu-exec.c

@@ -233,14 +233,18 @@ static void cpu_exec_step(CPUState *cpu)
     uint32_t flags;
 
     cpu_get_tb_cpu_state(env, &pc, &cs_base, &flags);
+    tb_lock();
     tb = tb_gen_code(cpu, pc, cs_base, flags,
                      1 | CF_NOCACHE | CF_IGNORE_ICOUNT);
     tb->orig_tb = NULL;
+    tb_unlock();
     /* execute the generated code */
     trace_exec_tb_nocache(tb, pc);
     cpu_tb_exec(cpu, tb);
+    tb_lock();
     tb_phys_invalidate(tb, -1);
     tb_free(tb);
+    tb_unlock();
 }
 
 void cpu_exec_step_atomic(CPUState *cpu)