LU-13915 ldiskfs: Avoid atomic operation while bitmap prefetch

LU-13915 ldiskfs: Avoid atomic operation while bitmap prefetch

It is expensive since test_and_set_bit is a atomic operation. So
use test_bit while bitmap prefetch to avoid call it frequently.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I2ff2c39f1dd3b351462ed66cbd3ebb36e6af4bea
Reviewed-on: https://review.whamcloud.com/39697
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <artem.blagodarenko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>