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/src/stream_interface.c b/src/stream_interface.c
index 9912df0..1ba509e 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -1185,7 +1185,7 @@
 	}
 
 	/* now we'll need a buffer */
-	if (!session_alloc_recv_buffer(si_sess(si), &ic->buf)) {
+	if (!session_alloc_recv_buffer(ic)) {
 		si->flags |= SI_FL_WAIT_ROOM;
 		goto end_recv;
 	}