[SKBUFF]: Free old skb properly in skb_morph

The skb_morph function only freed the data part of the dst skb, but leaked
the auxiliary data such as the netfilter fields.  This patch fixes this by
moving the relevant parts from __kfree_skb to skb_release_all and calling
it in skb_morph.

It also makes kfree_skbmem static since it's no longer called anywhere else
and it now no longer does skb_release_data.

Thanks to Yasuyuki KOZAKAI for finding this problem and posting a patch for
it.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>