[IPV6]: fix ipv6_saddr_score struct element

The scope element in the ipv6_saddr_score struct used in 
ipv6_dev_get_saddr() is an unsigned integer, but __ipv6_addr_src_scope() 
returns a signed integer (and can return -1).

Signed-off-by: Brian Haley <brian.haley@hp.com>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>