We now start to move across the JBD data structure's fields, from "innermost"
and outwards.

Start with journal_head.b_frozen_data, because the locking for this field was
partially implemented in jbd-010-b_committed_data-race-fix.patch.

It is protected by jbd_lock_bh_state().  We keep the lock_journal() and
spin_lock(&journal_datalist_lock) calls in place.  Later,
spin_lock(&journal_datalist_lock) is replaced by
spin_lock(&journal->j_list_lock).

Of course, this completion of the locking around b_frozen_data also puts a
lot of the locking for other fields in place.