net/ipv6: Remove unncessary check on f6i in fib6_check

Dan reported an imbalance in fib6_check on use of f6i and checking
whether it is null. Since fib6_check is only called if f6i is non-null,
remove the unnecessary check.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>