accel/tcg: Use more accurate max_insns for tb_overflow

accel/tcg: Use more accurate max_insns for tb_overflow

Write back the number of insns that we attempt to translate,
so that if we longjmp out we have a more accurate limit for
the next attempt.  This results in fewer restarts when some
limit is consumed by few instructions.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>