NFS: Ensure that mmapped pages remain stable during writeback

Ensure that nfs_vm_page_mkwrite() waits for the page writeback to
complete before the application is allowed to modify page
contents.
The main reason for wanting to do this in NFS is to ensure that the
server doesn't get confused if we have to resend the RPC request
due to a dropped/missed reply.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/file.c

@@ -530,6 +530,8 @@ static int nfs_vm_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
 	if (mapping != dentry->d_inode->i_mapping)
 		goto out_unlock;
 
+	wait_on_page_writeback(page);
+
 	pagelen = nfs_page_length(page);
 	if (pagelen == 0)
 		goto out_unlock;