ipv6: route: place a warning with duplicated string with correct extack

"IPv6: " prefix is already added by pr_fmt, no need to include
it again in the pr_warn() format.  The message predates extack
support, we can replace the whole thing with an extack message.

Suggested-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>