accel/tcg: Clear PAGE_WRITE before translation

accel/tcg: Clear PAGE_WRITE before translation

translate_insn() implementations fetch instruction bytes piecemeal,
which can cause qemu-user to generate inconsistent translations if
another thread modifies them concurrently [1].

Fix by making pages containing translated instruction non-writable
right before loading instruction bytes from them.

[1] https://lists.nongnu.org/archive/html/qemu-devel/2021-08/msg00644.html

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20210805204835.158918-1-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>