accel/tcg: fix race in cpu_exec_step_atomic (bug 1863025)

The bug describes a race whereby cpu_exec_step_atomic can acquire a TB
which is invalidated by a tb_flush before we execute it. This doesn't
affect the other cpu_exec modes as a tb_flush by it's nature can only
occur on a quiescent system. The race was described as:

B2. tcg_cpu_exec => cpu_exec => tb_find => tb_gen_code
B3. tcg_tb_alloc obtains a new TB

C3. TB obtained with tb_lookup__cpu_state or tb_gen_code
(same TB as B2)

A3. start_exclusive critical section entered
A4. do_tb_flush is called, TB memory freed/re-allocated
A5. end_exclusive exits critical section

B2. tcg_cpu_exec => cpu_exec => tb_find => tb_gen_code
B3. tcg_tb_alloc reallocates TB from B2

C4. start_exclusive critical section entered
C5. cpu_tb_exec executes the TB code that was free in A4

The simplest fix is to widen the exclusive period to include the TB
lookup. As a result we can drop the complication of checking we are in
the exclusive region before we end it.

Cc: Yifan <me@yifanlu.com>
Buglink: https://bugs.launchpad.net/qemu/+bug/1863025
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200214144952.15502-1-alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>