f2fs: fix to do f2fs_balance_fs in f2fs_map_blocks correctly

If we preallocate blocks with f2fs_reserve_blocks in f2fs_map_blocks, we
should call f2fs_balance_fs for checking and reclaiming space, fix it.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/data.c

@@ -783,6 +783,7 @@ int f2fs_map_blocks(struct inode *inode, struct f2fs_map_blocks *map,
 		err = reserve_new_blocks(&dn, prealloc);
 		if (err)
 			goto sync_out;
+		allocated = dn.node_changed;
 
 		map->m_len += dn.ofs_in_node - ofs_in_node;
 		if (prealloc && dn.ofs_in_node != last_ofs_in_node + 1) {