This patch adds a flag which indicates that the journal is frozen or not and
introduces two new functions jbd2_journal_freeze and jbd2_journal_thaw which
should be called when the F.S freezes and thaws respectively. 
A new handle can only be started now when the barrier count is 0 and when the
journal is not in a frozen state. While the journal is in a frozen state,
trying to start a new handle would put the process on wait queue. Thawing the
journal would wake up all the processes waiting on this wait queue.

I have lightly tested this patch. Sending it here for initial review. Please
do let me know your inputs. Thanks a lot!

Warm Regards,
Surbhi.