ext2: change return code to -ENOMEM when failing memory allocation

Change return code to -ENOMEM from -EINVAL when failing memory
allocation in fill_super(), meanwhile delete redundant initial
assignment of variable err.

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Signed-off-by: Jan Kara <jack@suse.cz>