rds: transport module should be auto loaded when transport is set

This enhancement auto loads transport module when the transport
is set via SO_RDS_TRANSPORT socket option.

Reviewed-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Reviewed-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Rao Shoaib <rao.shoaib@oracle.com>
Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>