Rooted, then flashed, now not able to root again /system is always read-only
I have posted the question on same link shown in question. Till that i tried everything format SD card totally, also tried different roms of ICS still it was not worked. 

Then as in reply i got on that forum saying install gingerbread rom then root it, use OTA root keeper and upgrade. 

So i did similar to that i installed gingerbread.

Then i formatted all using cwm. Then i installed new ICS refrec v2 rom. And then after i tried to root it. And it worked.
Hope so if anyone face the same problem they can downgrade to GB then again move to ICS to root it after flash if their favorite rom doesn't come rooted. :)