ipv6: copy prefsrc setting when copying route entry

commit c3968a85
('ipv6: RTA_PREFSRC support for ipv6 route source address selection')
added support for ipv6 prefsrc as an alternative to ipv6 addrlabels,
but it did not work because the prefsrc entry was not copied.

Cc: Daniel Walter <sahne@0x90.at>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>