udf: Avoid dir link count to go negative

If we are writing back inode of unlinked directory, its link count ends
up being (u16)-1. Although the inode is deleted, udf_iget() can load the
inode when NFS uses stale file handle and get confused.
Signed-off-by: NJan Kara <jack@suse.cz>

fs/udf/inode.c

@@ -1664,7 +1664,7 @@ static int udf_update_inode(struct inode *inode, int do_sync)
 		     FE_PERM_U_DELETE | FE_PERM_U_CHATTR));
 	fe->permissions = cpu_to_le32(udfperms);
 
-	if (S_ISDIR(inode->i_mode))
+	if (S_ISDIR(inode->i_mode) && inode->i_nlink > 0)
 		fe->fileLinkCount = cpu_to_le16(inode->i_nlink - 1);
 	else
 		fe->fileLinkCount = cpu_to_le16(inode->i_nlink);