Ext2: return ENOMEM rather than EIO if sb_getblk fails

As the only reason that sb_getblks fails is that allocation fails.
It will be better to use ENOMEM rather than EIO.

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Jan Kara <jack@suse.cz>