ipv4: accept u8 in IP_TOS ancillary data

In commit f02db315 ("ipv4: IP_TOS and IP_TTL can be specified as
ancillary data") Francesco added IP_TOS values specified as integer.

However, kernel sends to userspace (at recvmsg() time) an IP_TOS value
in a single byte, when IP_RECVTOS is set on the socket.

It can be very useful to reflect all ancillary options as given by the
kernel in a subsequent sendmsg(), instead of aborting the sendmsg() with
EINVAL after Francesco patch.

So this patch extends IP_TOS ancillary to accept an u8, so that an UDP
server can simply reuse same ancillary block without having to mangle
it.

Jesper can then augment
https://github.com/netoptimizer/network-testing/blob/master/src/udp_example02.c
to add TOS reflection ;)

Fixes: f02db315 ("ipv4: IP_TOS and IP_TTL can be specified as ancillary data")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Francesco Fusco <ffusco@redhat.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -284,9 +284,12 @@ int ip_cmsg_send(struct sock *sk, struct msghdr *msg, struct ipcm_cookie *ipc,
 			ipc->ttl = val;
 			break;
 		case IP_TOS:
-			if (cmsg->cmsg_len != CMSG_LEN(sizeof(int)))
-				return -EINVAL;
+			if (cmsg->cmsg_len == CMSG_LEN(sizeof(int)))
 				val = *(int *)CMSG_DATA(cmsg);
+			else if (cmsg->cmsg_len == CMSG_LEN(sizeof(u8)))
+				val = *(u8 *)CMSG_DATA(cmsg);
+			else
+				return -EINVAL;
 			if (val < 0 || val > 255)
 				return -EINVAL;
 			ipc->tos = val;