[NET]: Make the sk_clone() lighter

The sk_prot_alloc() already performs all the stuff needed by the
sk_clone(). Besides, the sk_prot_alloc() requires almost twice
less arguments than the sk_alloc() does, so call the sk_prot_alloc()
saving the stack a bit.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>