Ext3: add necessary check in case IO error happens

As we know io error may happen when the function 'sb_getblk'
is called.Add necessary check for it

The patch also fix a coding style problem.

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