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>
diff --git a/net/rds/rds.h b/net/rds/rds.h
index 7e52b92..0d8f67c 100644
--- a/net/rds/rds.h
+++ b/net/rds/rds.h
@@ -574,6 +574,7 @@ struct rds_transport {
 	void (*free_mr)(void *trans_private, int invalidate);
 	void (*flush_mrs)(void);
 	bool (*t_unloading)(struct rds_connection *conn);
+	u8 (*get_tos_map)(u8 tos);
 };
 
 /* Bind hash table key length.  It is the sum of the size of a struct