[NET]: Move the get_net() from sock_copy()

The sock_copy() is supposed to just clone the socket. In a perfect
world it has to be just memcpy, but we have to handle the security
mark correctly. All the extra setup must be performed in sk_clone() 
call, so move the get_net() into more proper place.

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