[arch/linux] udp: only read up to available buffer space (#1622)

In the udp read, the code first checks if there is space available in the transport buffer but then reads the maximum amount of data for the transport buffer, I simply changed it such that it reads up to the available limit of the transport buffer.

sw/airborne/arch/linux/mcu_periph/udp_arch.c

@@ -121,7 +121,7 @@ void udp_receive(struct udp_periph *p)
   }
 
   socklen_t slen = sizeof(struct sockaddr_in);
-  ssize_t byte_read = recvfrom(sock->sockfd, buf, UDP_RX_BUFFER_SIZE, MSG_DONTWAIT,
+  ssize_t byte_read = recvfrom(sock->sockfd, buf, available, MSG_DONTWAIT,
                                (struct sockaddr *)&sock->addr_in, &slen);
 
   pthread_mutex_lock(&udp_mutex);