f2fs: fix to tag FIEMAP_EXTENT_MERGED in f2fs_fiemap()

f2fs does not natively support extents in metadata, 'extent' in f2fs
is used as a virtual concept, so in f2fs_fiemap() interface, it needs
to tag FIEMAP_EXTENT_MERGED flag to indicated the extent status is a
result of merging.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -1964,6 +1964,7 @@ int f2fs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
 	}
 
 	if (size) {
+		flags |= FIEMAP_EXTENT_MERGED;
 		if (IS_ENCRYPTED(inode))
 			flags |= FIEMAP_EXTENT_DATA_ENCRYPTED;