lib: return error code if bf_send() fails

bf_send() calls bf_send_request() and bf_recv_response(), both return
non-zero on failure. While the success check is performed on their
return value, once the error is logged errno is returned by bf_send().
Errno is not set by bf_send_request() not bf_recv_response(),
effectively returning 0 from bf_send() and hiding the error.

src/libbpfilter/generic.c

@@ -35,11 +35,11 @@ int bf_send(const struct bf_request *request, struct bf_response **response)
 
     r = bf_send_request(fd, request);
     if (r < 0)
-        return bf_err(errno, "bpfilter: failed to send request to the daemon");
+        return bf_err(r, "bpfilter: failed to send request to the daemon");
 
     r = bf_recv_response(fd, response);
     if (r < 0) {
-        return bf_err(errno,
+        return bf_err(r,
                       "bpfilter: failed to receive response from the daemon");
     }