[5.4] route6/fib6 : correct the program not to do panic even if a kmem_cache_create is failed

Issue

• [5.4] route6/fib6 : correct the program not to do panic even if a kmem_cache_create is failed
• If the kmem_cache_create 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, we don't want to panic instead  of just failing to initialize the protocol.