jfs_sync_fs was implemented using the same code as the unmount code to flush
the journal and wait for the journal to quiesce.  Since jfs_sync_fs may  be
called while the volume is under heavy use, we can end up waiting
indefinately.  Code in jfs_flush_journal meant to detect a hang at unmount
time may be triggered in this case causing a trap.  This patch changes
jfs_sync_fs to only wait until the most recent transaction has been
commited to disk, rather than waiting until the commit queue is empty.