RDS: add receive message trace used by application

Socket option to tap receive path latency in various stages
in nano seconds. It can be enabled on selective sockets using
using SO_RDS_MSG_RXPATH_LATENCY socket option. RDS will return
the data to application with RDS_CMSG_RXPATH_LATENCY in defined
format. Scope is left to add more trace points for future
without need of change in the interface.

Reviewed-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>

net/rds/ib_recv.c

@@ -911,8 +911,12 @@ static void rds_ib_process_recv(struct rds_connection *conn,
 		ic->i_ibinc = ibinc;
 
 		hdr = &ibinc->ii_inc.i_hdr;
+		ibinc->ii_inc.i_rx_lat_trace[RDS_MSG_RX_HDR] =
+				local_clock();
 		memcpy(hdr, ihdr, sizeof(*hdr));
 		ic->i_recv_data_rem = be32_to_cpu(hdr->h_len);
+		ibinc->ii_inc.i_rx_lat_trace[RDS_MSG_RX_START] =
+				local_clock();
 
 		rdsdebug("ic %p ibinc %p rem %u flag 0x%x\n", ic, ibinc,
 			 ic->i_recv_data_rem, hdr->h_flags);