net/ipv6: Put lwtstate when destroying fib6_info

Prior to the introduction of fib6_info lwtstate was managed by the dst
code. With fib6_info releasing lwtstate needs to be done when the struct
is freed.

Fixes: 93531c67 ("net/ipv6: separate handling of FIB entries from dst based routes")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>