rds: add transport specific tos_map hook

RDMA transport maps user tos to underline virtual lanes(VL)
for IB or DSCP values. RDMA CM transport abstract thats for
RDS. TCP transport makes use of default priority 0 and maps
all user tos values to it.

Reviewed-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
[yanjun.zhu@oracle.com: Adapted original patch with ipv6 changes]
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>