f2fs: always assume that the device is idle under gc_urgent

This allows more aggressive discards and balancing job to be done
under gc_urgent.

Signed-off-by: Park Ju Hyung <qkrwngud825@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>