On Sun, Aug 28, 2016 at 11:34:06AM +0800, Eli Cooper wrote:

According to RFC 1885 2.2(c), the source address of ICMPv6
errors in response to forwarded packets should be set to the
unicast address of the forwarding interface in order to be helpful
in diagnosis.

FWIW, this behaviour has been deprecated ten years ago by RFC 4443:
"The address SHOULD be chosen according to the rules that would be used
 to select the source address for any other packet originated by the
 node, given the destination address of the packet."

The door is left open for other address selection algorithms but, IMHO,
changing kernel's behaviour is better justified by real use cases
than by obsolete RFCs.