selftests/net: Interpret UDP_GRO cmsg data as an int value

Data passed to user-space with a (SOL_UDP, UDP_GRO) cmsg carries an
int (see udp_cmsg_recv), not a u16 value, as strace confirms:

  recvmsg(8, {msg_name=...,
              msg_iov=[{iov_base="\0\0..."..., iov_len=96000}],
              msg_iovlen=1,
              msg_control=[{cmsg_len=20,         <-- sizeof(cmsghdr) + 4
                            cmsg_level=SOL_UDP,
                            cmsg_type=0x68}],    <-- UDP_GRO
                            msg_controllen=24,
                            msg_flags=0}, 0) = 11200

Interpreting the data as an u16 value won't work on big-endian platforms.
Since it is too late to back out of this API decision [1], fix the test.

[1]: https://lore.kernel.org/netdev/20230131174601.203127-1-jakub@cloudflare.com/

Fixes: 3327a9c ("selftests: add functionals test for UDP GRO")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: jsitnicki

tools/testing/selftests/net/udpgso_bench_rx.c

@@ -214,11 +214,10 @@ static void do_verify_udp(const char *data, int len)
 
 static int recv_msg(int fd, char *buf, int len, int *gso_size)
 {
-	char control[CMSG_SPACE(sizeof(uint16_t))] = {0};
+	char control[CMSG_SPACE(sizeof(int))] = {0};
 	struct msghdr msg = {0};
 	struct iovec iov = {0};
 	struct cmsghdr *cmsg;
-	uint16_t *gsosizeptr;
 	int ret;
 
 	iov.iov_base = buf;
@@ -237,8 +236,7 @@ static int recv_msg(int fd, char *buf, int len, int *gso_size)
 		     cmsg = CMSG_NXTHDR(&msg, cmsg)) {
 			if (cmsg->cmsg_level == SOL_UDP
 			    && cmsg->cmsg_type == UDP_GRO) {
-				gsosizeptr = (uint16_t *) CMSG_DATA(cmsg);
-				*gso_size = *gsosizeptr;
+				*gso_size = *(int *)CMSG_DATA(cmsg);
 				break;
 			}
 		}