If a multicast packet gets looped back, the sending
socket can hang if a local read just sits and does
not empty its receive queue.

The problem is that when an SKB clone is freed up,
the destructor is only invoked for the head SKB when
there is a fraglist (which is created for fragmentation).

The solution is to account the fragment list SKB lengths
in the top-level head SKB, then it all works out.
Signed-off-by: David S. Miller <davem@redhat.com>