MAINTAINERS: Update entries of ext2 and ext3

I often handle ext2 and ext3 patches these days so add myself to MAINTAINERS.

CC: Andrew Morton <akpm@linux-foundation.org>
CC: Andreas Dilger <andreas.dilger@oracle.com>
CC: linux-ext4@vger.kernel.org
Signed-off-by: Jan Kara <jack@suse.cz>