net: factor out a helper to decrement the skb refcount

The same code is replicated in 3 different places; move it to a
common helper.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>