f2fs: fix last_lblock check in check_swap_activate_fast

Because page_no < sis->max guarantees that the while loop break out
normally, the wrong check contidion here doesn't cause a problem.

Signed-off-by: Huang Jianan <huangjianan@oppo.com>
Signed-off-by: Guo Weichao <guoweichao@oppo.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>