f2fs: allow readpages with NULL file pointer

Keep in line with the other Linux file system implementations
since page_cache_sync_readahead supports NULL file pointer,
and thus we can readahead data by f2fs itself without file opening
(something like the btrfs behavior).

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>