net/rds: Add RDS6_INFO_SOCKETS and RDS6_INFO_RECV_MESSAGES options

Add support of the socket options RDS6_INFO_SOCKETS and
RDS6_INFO_RECV_MESSAGES which update the RDS_INFO_SOCKETS and
RDS_INFO_RECV_MESSAGES options respectively.  The old options work
for IPv4 sockets only.

Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>