[IPV6] route6/fib6: Don't panic a kmem_cache_create.

If the kmem_cache_creation fails, the kernel will panic. It is
acceptable if the system is booting, but if the ipv6 protocol is
compiled as a module and it is loaded after the system has booted, do
we want to panic instead of just failing to initialize the protocol ?

The init function is now returning an error and this one is checked
for protocol initialization. So the ipv6 protocol will safely fails.

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>