[XFS] Fix dbflush panic in xfs_qm_sync.

The recent behaviour layer removal dropped the check for quotas that have
been requested at mount time but have subsequently been turned off. This
results in a panic when accessing m_quotainfo which has been freed.

This patch adds the check originally made by xfs_qm_syncall() to
xfs_qm_sync().

SGI-PV: 969769
SGI-Modid: xfs-linux-melb:xfs-kern:29908a
Signed-off-by: Donald Douwsma <donaldd@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>

fs/xfs/quota/xfs_qm.c

@@ -1008,6 +1008,9 @@ xfs_qm_sync(
 	boolean_t	nowait;
 	int		error;
 
+	if (! XFS_IS_QUOTA_ON(mp))
+		return 0;
+
 	restarts = 0;
 	/*
 	 * We won't block unless we are asked to.