ipv6: add ipv6_dev_find()

This is to add an ip_dev_find like function for ipv6, used to find
the dev by saddr.

It will be used by TIPC protocol. So also export it.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>