xfs: reimplement fdatasync support

Add an in-memory only flag to say we logged timestamps only, and use it to
check if fdatasync can optimize away the log force.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 78d8b02..54a67dd 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -197,8 +197,11 @@
 	 * to flush the log up to the latest LSN that touched the inode.
 	 */
 	xfs_ilock(ip, XFS_ILOCK_SHARED);
-	if (xfs_ipincount(ip))
-		lsn = ip->i_itemp->ili_last_lsn;
+	if (xfs_ipincount(ip)) {
+		if (!datasync ||
+		    (ip->i_itemp->ili_fields & ~XFS_ILOG_TIMESTAMP))
+			lsn = ip->i_itemp->ili_last_lsn;
+	}
 	xfs_iunlock(ip, XFS_ILOCK_SHARED);
 
 	if (lsn)