Ext3: return ENOMEM rather than EIO if sb_getblk fails

It will be better to use ENOMEM rather than EIO, because the only
reason that sb_getblk fails is that allocation fails.

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