net/rds: Declare SO_RDS_TRANSPORT and RDS_TRANS_* constants in uapi/linux/rds.h

User space applications that desire to explicitly select the
underlying transport for a PF_RDS socket may do so by using the
SO_RDS_TRANSPORT socket option at the SOL_RDS level before bind().
The integer argument provided to the socket option would be one
of the RDS_TRANS_* values, e.g., RDS_TRANS_TCP. This commit exports
the constant values need by such applications via <linux/rds.h>

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>