Increase GRO segments to 64

Unless the sender and the receiver agree on the exact size of the
datagrams they're going to exchange, there's no way to know the GRO
segment size before calling recvmmsg(). This makes it impossible to
calculate how big the receive buffer should be before calling
recvmmsg(), potentially leading to truncated segments if it isn't a
multiple of the GRO segment size.

Bumping gro::gro_segments() to the max allowed by the kernel
(UDP_GRO_CNT_MAX = 64), and assuming that get_max_udp_payload_size() is
a valid upper bound for incoming datagrams, ensures that the receive
window we allocate is always large enough.

Author: alessandrod

quinn-udp/src/unix.rs

@@ -645,7 +645,9 @@ mod gro {
         };
 
         if rc != -1 {
-            10
+            // As defined in net/ipv4/udp_offload.c
+            // #define UDP_GRO_CNT_MAX 64
+            64
         } else {
             1
         }