cifs: Fix setting of zero_point after DIO write

At the moment, at the end of a DIO write, cifs calls netfs_resize_file() to
adjust the size of the file if it needs it. This will reduce the
zero_point (the point above which we assume a read will just return zeros)
if it's more than the new i_size, but won't increase it.

With DIO writes, however, we definitely want to increase it as we have
clobbered the local pagecache and then written some data that's not
available locally.

Fix cifs to make the zero_point above the end of a DIO or unbuffered write.

This fixes corruption seen occasionally with the generic/708 xfs-test. In
that case, the read-back of some of the written data is being
short-circuited and replaced with zeroes.

Fixes: 3ee1a1fc3981 ("cifs: Cut over to using netfslib")
Cc: [e-mail]
Reported-by: Steve French <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Paulo Alcantara (Red Hat) <[e-mail]>
cc: Jeff Layton <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
Signed-off-by: Steve French <[e-mail]>

fs/smb/client/file.c

···
 				      bool was_async)
 {
 	struct netfs_io_request *wreq = wdata->rreq;
-	loff_t new_server_eof;
+	struct netfs_inode *ictx = netfs_inode(wreq->inode);
+	loff_t wrend;
 
 	if (result > 0) {
-		new_server_eof = wdata->subreq.start + wdata->subreq.transferred + result;
+		wrend = wdata->subreq.start + wdata->subreq.transferred + result;
 
-		if (new_server_eof > netfs_inode(wreq->inode)->remote_i_size)
-			netfs_resize_file(netfs_inode(wreq->inode), new_server_eof, true);
+		if (wrend > ictx->zero_point &&
+		    (wdata->rreq->origin == NETFS_UNBUFFERED_WRITE ||
+		     wdata->rreq->origin == NETFS_DIO_WRITE))
+			ictx->zero_point = wrend;
+		if (wrend > ictx->remote_i_size)
+			netfs_resize_file(ictx, wrend, true);
 	}
 
 	netfs_write_subrequest_terminated(&wdata->subreq, result, was_async);