NFSv4.1: Add a helper pnfs_commit_and_return_layout

In order to be able to safely return the layout in nfs4_proc_setattr,
we need to block new uses of the layout, wait for all outstanding
users of the layout to complete, commit the layout and then return it.

This patch adds a helper in order to do all this safely.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Boaz Harrosh <bharrosh@panasas.com>
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 6ccdd4f..26431cf 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -2632,7 +2632,7 @@
 	int status;
 
 	if (pnfs_ld_layoutret_on_setattr(inode))
-		pnfs_return_layout(inode);
+		pnfs_commit_and_return_layout(inode);
 
 	nfs_fattr_init(fattr);