kernel: net: fib: fib6_add: potential NULL pointer dereference

The fib6add function in net/ipv6/ip6fib.c in the Linux kernel before 3.11.5 does not properly implement error-code encoding, which allows local users to cause a denial of service NULL pointer dereference and system crash by leveraging the CAPNETADMIN capability for an IPv6 SIOCADDRT ioctl call...