ipv6: fix memory leak on dst->_metrics

When dst->_metrics and f6i->fib6_metrics share the same memory, both
take reference count on the dst_metrics structure. However, when dst is
destroyed, ip6_dst_destroy() only invokes dst_destroy_metrics_generic()
which does not take care of READONLY metrics and does not release refcnt.
This causes memory leak.
Similar to ipv4 logic, the fix is to properly release refcnt and free
the memory space pointed by dst->_metrics if refcnt becomes 0.

Fixes: 93531c67

 ("net/ipv6: separate handling of FIB entries from dst based routes")
Reported-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>