MINOR: channel/buffer: replace b_{adv,rew} with c_{adv,rew}

These ones manipulate the output data count which will be specific to
the channel soon, so prepare the call points to use the channel only.
The b_* functions are now unused and were removed.
diff --git a/src/flt_trace.c b/src/flt_trace.c
index fa26de1..f088340 100644
--- a/src/flt_trace.c
+++ b/src/flt_trace.c
@@ -465,9 +465,9 @@
 		   FLT_NXT(filter, msg->chn), FLT_FWD(filter, msg->chn), ret);
 
 	if (conf->hexdump) {
-		b_adv(msg->chn->buf, FLT_FWD(filter, msg->chn));
+		c_adv(msg->chn, FLT_FWD(filter, msg->chn));
 		trace_hexdump(msg->chn->buf, ret);
-		b_rew(msg->chn->buf, FLT_FWD(filter, msg->chn));
+		c_rew(msg->chn, FLT_FWD(filter, msg->chn));
 	}
 
 	if ((ret != len) ||
@@ -515,9 +515,9 @@
 		   FLT_FWD(filter, chn), ret);
 
 	if (conf->hexdump) {
-		b_adv(chn->buf, FLT_FWD(filter, chn));
+		c_adv(chn, FLT_FWD(filter, chn));
 		trace_hexdump(chn->buf, ret);
-		b_rew(chn->buf, FLT_FWD(filter, chn));
+		c_rew(chn, FLT_FWD(filter, chn));
 	}
 
 	if (ret != len)