net/key/af_key.c: add missing kfree_skb

At the point of this error-handling code, alloc_skb has succeded, so free
the resulting skb by jumping to the err label.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>