JFS currently writes a journal page as any transaction is committed, as
long as there is no current journal I/O.  This results in a lot of partial
journal pages being written, even if no threads are waiting for the commit
to complete.  This patch avoids these partial page writes if the transaction
being committed is asynchronous (lazy).  It a couple places, we need to
make sure that the group commit is performed to flush all transactions to
disk.