[IPV6]: Make fib6_init to return an error code.

If there is an error in the initialization function, nothing is
followed up to the caller. So I add a return value to be set for the
init function.

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>