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: Jan Kara <jack@suse.cz>
diff --git a/fs/udf/inode.c b/fs/udf/inode.c
index 236cd48..e86f9b6 100644
--- a/fs/udf/inode.c
+++ b/fs/udf/inode.c
@@ -1664,7 +1664,7 @@
 		     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);