RDS: Implement atomic operations

Implement a CMSG-based interface to do FADD and CSWP ops.

Alter send routines to handle atomic ops.

Add atomic counters to stats.

Add xmit_atomic() to struct rds_transport

Inline rds_ib_send_unmap_rdma into unmap_rm

Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/stats.c

@@ -75,6 +75,8 @@ static const char *const rds_stat_names[] = {
 	"cong_update_received",
 	"cong_send_error",
 	"cong_send_blocked",
+	"s_atomic_cswp",
+	"s_atomic_fadd",
 };
 
 void rds_stats_info_copy(struct rds_info_iterator *iter,