[IPV6]: Fix dangling references on error in fib6_add().

Fixes bugzilla #8895

If a super-tree leaf has 'rt' assigned to it and we
get an error from fib6_add_rt2node(), we'll leave
a reference to 'rt' in pn->leaf and then do an
unconditional dst_free().

We should prune such references.

Based upon a report by Vincent Perrier.

Signed-off-by: David S. Miller <davem@davemloft.net>