On munmap() zap_pte_range() is called which dirties the PTE dirty pages as
Toshiyuki pointed out.

zap_pte_range()
  mapping->a_ops->set_page_dirty (= ext4_journalled_set_page_dirty)  

So, I think that it is here that we should do the checking for a ext4 F.S
frozen state and also prevent a parallel ext4 F.S freeze from happening.

Attaching a patch for initial review. Please do let me know your thoughts! 

Thanks a lot!

Warm Regards,
Surbhi.