net: ipv6: Generate random IID for addresses on RAWIP devices

RAWIP devices such as rmnet do not have a hardware address and
instead require the kernel to generate a random IID for the
IPv6 addresses.

Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>