xfs: only issues a cache flush on unmount if barriers are enabled

Currently we unconditionally issue a flush from xfs_free_buftarg, but
since 2.6.29-rc1 this gives a warning in the style of

	end_request: I/O error, dev vdb, sector 0

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Felix Blyakher <felixb@sgi.com>
diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
index cb329ed..aa1016b 100644
--- a/fs/xfs/linux-2.6/xfs_buf.c
+++ b/fs/xfs/linux-2.6/xfs_buf.c
@@ -34,6 +34,12 @@
 #include <linux/backing-dev.h>
 #include <linux/freezer.h>
 
+#include "xfs_sb.h"
+#include "xfs_inum.h"
+#include "xfs_ag.h"
+#include "xfs_dmapi.h"
+#include "xfs_mount.h"
+
 static kmem_zone_t *xfs_buf_zone;
 STATIC int xfsbufd(void *);
 STATIC int xfsbufd_wakeup(int, gfp_t);
@@ -1435,10 +1441,12 @@
 
 void
 xfs_free_buftarg(
-	xfs_buftarg_t		*btp)
+	struct xfs_mount	*mp,
+	struct xfs_buftarg	*btp)
 {
 	xfs_flush_buftarg(btp, 1);
-	xfs_blkdev_issue_flush(btp);
+	if (mp->m_flags & XFS_MOUNT_BARRIER)
+		xfs_blkdev_issue_flush(btp);
 	xfs_free_bufhash(btp);
 	iput(btp->bt_mapping->host);