JFS: Flush dirty metadata to disk when remounting from read-write
to read-only.  Also fix umount ordering to make sure metadata is
written before journal closed.

With Andrew Morton's recent changes, JFS is no longer writing much
of its dirty metadata when remounting from read-write to read-only.
This causes severe file system corruption.  A JFS root file system
will be corrupted on shutdown.

This patch fixes the problem by explicitly writing the dirty metadata
before the journal is closed.  It also fixes the ordering so that
the dirty metadata is completely written before the journal is closed
for the normal unmount case as well.