[NET_SCHED]: turn PSCHED_TDIFF_SAFE into inline function

Also rename to psched_tdiff_bounded.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/sch_tbf.c b/net/sched/sch_tbf.c
index 626ce96..da9f40e 100644
--- a/net/sched/sch_tbf.c
+++ b/net/sched/sch_tbf.c
@@ -201,7 +201,7 @@
 
 		PSCHED_GET_TIME(now);
 
-		toks = PSCHED_TDIFF_SAFE(now, q->t_c, q->buffer);
+		toks = psched_tdiff_bounded(now, q->t_c, q->buffer);
 
 		if (q->P_tab) {
 			ptoks = toks + q->ptokens;