Install open ssh and ifile with Cydia on the ipad.

start ifile on the ipad
and browse to /etc
then select the master.passwd file
and change the owner from root to the user mobile

go back to the file properties
and hit the change button

Next setup a putty or freexer connection from a computer to the ipad by ip address
logon with user mobile and default password alpine

Make a backup of the master.passwd file;
cp /etc/master.passwd /tmp/master.passwd.bck

Use a file editor or winscp to change the master.passwd root hash entry with the hash of the mobile user hash.

Change the owner rights back to root with ifile app
and you are able to logon with username root
and the default password alpine again.