MEDIUM: session: simplify receive buffer allocator to only use the channel

Now that we can get the session from the channel, let's simplify the
prototype of session_alloc_recv_buffer() to only require the channel.
Both the caller and the function are now simplified.
diff --git a/include/proto/session.h b/include/proto/session.h
index fd62dfd..2dc3d83 100644
--- a/include/proto/session.h
+++ b/include/proto/session.h
@@ -60,7 +60,7 @@
 static inline void session_offer_buffers();
 int session_alloc_work_buffer(struct session *s);
 void session_release_buffers(struct session *s);
-int session_alloc_recv_buffer(struct session *s, struct buffer **buf);
+int session_alloc_recv_buffer(struct channel *chn);
 
 /* sets the stick counter's entry pointer */
 static inline void stkctr_set_entry(struct stkctr *stkctr, struct stksess *entry)