Almost everywhere where JBD removes a buffer from the transaction lists the
caller then nulls out jh->b_transaction.  Sometimes, the caller does that
without holding the locks which are defined to protect b_transaction.  This
makes me queazy.

So change things so that __journal_unfile_buffer() nulls out b_transaction
inside both j_list_lock and jbd_lock_bh_state().

It cleans things up a bit, too.