[IPV6]: Make af_inet6 to check ip6_route_init return value.

The af_inet6 initialization function does not check the return code of
the route initilization, so if something goes wrong, the protocol
initialization will continue anyway.  This patch takes into account
the modification made in the different route's initialization
subroutines to check the return value and to make the protocol
initialization to fail.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>