rds: Enable RDS IPv6 support

rds: Enable RDS IPv6 support

This patch enables RDS to use IPv6 addresses.  There are many data
structures (RDS socket options) used by RDS apps which use a 32 bit
integer to store IP address. To support IPv6, struct in6_addr needs to
be used. To ensure backward compatibility, a new data structure is
introduced for each of those data structures which use a 32 bit
integer to represent an IP address. And new socket options are
introduced to use those new structures. This means that existing apps
should work without a problem with the new RDS module.  For apps which
want to use IPv6, those new data structures and socket options can be
used. IPv4 mapped address is used to represent IPv4 address in the new
data structures.

RDS/RDMA/IB uses a private data (struct rds_ib_connect_private)
exchange between endpoints at RDS connection establishment time to
support RDMA. This private data exchange uses a 32 bit integer to
represent an IP address. This needs to be changed in order to support
IPv6. A new private data struct rds6_ib_connect_private is introduced
to handle this. To ensure backward compatibility, an IPv6 capable RDS
stack uses another RDMA listener port (RDS_CM_PORT which is 16385,
the same value as the RDS/TCP listener port number) to accept IPv6
connection. And it continues to use the original RDS_PORT for IPv4
RDS connections. When it needs to communicate with an IPv6 peer, it
uses the RDS_CM_PORT to send the connection set up request.

Orabug: 25410192

Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>