ipv6: export IP6_RT_PRIO_* to userland

The kernel uses some default metric when routes are managed. For example, a
static route added with a metric set to 0 is inserted in the kernel with
metric 1024 (IP6_RT_PRIO_USER).
It is useful for routing daemons to know these values, to be able to set routes
without interfering with what the kernel does.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>