net/ipv6: Remove rt6i_prefsrc

After the conversion to fib6_info, rt6i_prefsrc has a single user that
reads the value and otherwise it is only set. The one reader can be
converted to use rt->from so rt6i_prefsrc can be removed, reducing
rt6_info by another 20 bytes.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>