ext3.txt: update the links in the section "useful links" to the latest ones

In Documentation/filesystems/ext3.txt, the section "useful links"
provides two links which link to ibm developerworks articles. While
the second one can be redirected to the latest one, the first one
   http://www.ibm.com/developerworks/library/l-fs7.html


fails to be redirected.

Update the 2 links to the latest ones.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>