net: net_assign_generic() fix

memcpy() should take into account size of pointers,
not only number of pointers to copy.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>