f2fs: fix wrong block count instead of bytes

We should convert cur_lblock, a block count, to bytes for len.

Fixes: af4b6b8e ("f2fs: introduce check_swap_activate_fast()")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>