ipv6: Use BUG_ON

The semantic patch that makes this transformation is as follows:

// <smpl>
@@ expression e; @@
-if (e) BUG();
+BUG_ON(e);
// </smpl>

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>