MINOR: buffer: only use b_free to release buffers

We don't call pool_free2(pool2_buffers) anymore, we only call b_free()
to do the job. This ensures that we can start to centralize the releasing
of buffers.
diff --git a/include/common/buffer.h b/include/common/buffer.h
index 52565dd..2c992c7 100644
--- a/include/common/buffer.h
+++ b/include/common/buffer.h
@@ -409,6 +409,13 @@
 	return *buf;
 }
 
+/* Releases buffer *buf.
+ */
+static inline void b_free(struct buffer **buf)
+{
+	pool_free2(pool2_buffer, *buf);
+}
+
 #endif /* _COMMON_BUFFER_H */
 
 /*
diff --git a/src/peers.c b/src/peers.c
index af413c6..5371e49 100644
--- a/src/peers.c
+++ b/src/peers.c
@@ -1302,7 +1302,7 @@
  out_fail_rep_buf:
 	pool_free2(pool2_channel, s->rep);
  out_fail_rep:
-	pool_free2(pool2_buffer, s->req->buf);
+	b_free(&s->req->buf);
  out_fail_req_buf:
 	pool_free2(pool2_channel, s->req);
  out_fail_req:
diff --git a/src/session.c b/src/session.c
index 6770ee4..ebe992b 100644
--- a/src/session.c
+++ b/src/session.c
@@ -565,11 +565,11 @@
 
 	/* Error unrolling */
  out_free_rep_buf:
-	pool_free2(pool2_buffer, s->rep->buf);
+	b_free(&s->rep->buf);
  out_free_rep:
 	pool_free2(pool2_channel, s->rep);
  out_free_req_buf:
-	pool_free2(pool2_buffer, s->req->buf);
+	b_free(&s->req->buf);
  out_free_req:
 	pool_free2(pool2_channel, s->req);
  out_free_task:
@@ -618,8 +618,8 @@
 	if (s->rep->pipe)
 		put_pipe(s->rep->pipe);
 
-	pool_free2(pool2_buffer, s->req->buf);
-	pool_free2(pool2_buffer, s->rep->buf);
+	b_free(&s->req->buf);
+	b_free(&s->rep->buf);
 
 	pool_free2(pool2_channel, s->req);
 	pool_free2(pool2_channel, s->rep);