MEDIUM: connection: make xprt->rcv_buf() use size_t for the count

Just like we have a size_t for xprt->snd_buf(), we adjust to use size_t
for rcv_buf()'s count argument and return value. It also removes the
ambiguity related to the possibility to see a negative value there.
diff --git a/include/types/connection.h b/include/types/connection.h
index f9757e5..69d7394 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -268,7 +268,7 @@
  * and the other ones are used to setup and release the transport layer.
  */
 struct xprt_ops {
-	int  (*rcv_buf)(struct connection *conn, struct buffer *buf, int count); /* recv callback */
+	size_t (*rcv_buf)(struct connection *conn, struct buffer *buf, size_t count); /* recv callback */
 	size_t (*snd_buf)(struct connection *conn, const struct buffer *buf, size_t count, int flags); /* send callback */
 	int  (*rcv_pipe)(struct connection *conn, struct pipe *pipe, unsigned int count); /* recv-to-pipe callback */
 	int  (*snd_pipe)(struct connection *conn, struct pipe *pipe); /* send-to-pipe callback */