net: ipv6: use the new fib6_nh_release_dsts helper in fib6_nh_release

We can remove a bit of code duplication by reusing the new
fib6_nh_release_dsts helper in fib6_nh_release. Their only difference is
that fib6_nh_release's version doesn't use atomic operation to swap the
pointers because it assumes the fib6_nh is no longer visible, while
fib6_nh_release_dsts can be used anywhere.

Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>