nfs41: change PNFS_LAYOUTRET_ON_SETATTR to only return on truncation to smaller size

Both blocks layout and objects layout want to use it to avoid CB_LAYOUTRECALL
but that should only happen if client is doing truncation to a smaller size.
For other cases, we let server decide if it wants to recall client's layouts.
Change PNFS_LAYOUTRET_ON_SETATTR to follow the logic and not to send
layoutreturn unnecessarily.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Boaz Harrosh <boaz@plexistor.com>
Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/fs/nfs/pnfs.h b/fs/nfs/pnfs.h
index b10f12f..693ce42 100644
--- a/fs/nfs/pnfs.h
+++ b/fs/nfs/pnfs.h
@@ -69,7 +69,8 @@
 };
 
 enum layoutdriver_policy_flags {
-	/* Should the pNFS client commit and return the layout upon a setattr */
+	/* Should the pNFS client commit and return the layout upon truncate to
+	 * a smaller size */
 	PNFS_LAYOUTRET_ON_SETATTR	= 1 << 0,
 	PNFS_LAYOUTRET_ON_ERROR		= 1 << 1,
 	PNFS_READ_WHOLE_PAGE		= 1 << 2,