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: NGao Xiang <gaoxiang25@huawei.com>
Reviewed-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1334,7 +1334,7 @@ static int f2fs_read_data_pages(struct file *file,
 			struct address_space *mapping,
 			struct list_head *pages, unsigned nr_pages)
 {
-	struct inode *inode = file->f_mapping->host;
+	struct inode *inode = mapping->host;
 	struct page *page = list_last_entry(pages, struct page, lru);
 
 	trace_f2fs_readpages(inode, page, nr_pages);