net: addr IFLA_OPERSTATE to netlink message for ipv6 ifinfo

This is useful information to include in ipv6 netlink messages that
report interface information.  IFLA_OPERSTATE is already included in
ipv4 messages, but missing for ipv6.  This closes that gap.

Signed-off-by: Andy Gospodarek <gospo@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>