cpu-exec: rename have_tb_lock to acquired_tb_lock in tb_find

cpu-exec: rename have_tb_lock to acquired_tb_lock in tb_find

Reusing the have_tb_lock name, which is also defined in translate-all.c,
makes code reviewing unnecessarily harder.

Avoid potential confusion by renaming the local have_tb_lock variable
to something else.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>