[IPV6]: Replace using the magic constant "1024" with IP6_RT_PRIO_USER for fc_metric.

This patch replaces the explicit usage of the magic constant "1024"
with IP6_RT_PRIO_USER in the IPV6 tree.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>