f2fs: check if inmem_pages list is empty correctly

`cur' will never be NULL, we should check inmem_pages list instead.

Signed-off-by: Sheng Yong <shengyong1@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>