modem: simcom-sim7080: do not send fragmented data as multiple datagrams

Check if there are multiple non-empty data fragments passed to sendmsg()
function. If positive, then set EMSGSIZE errno and return -1, as that case
is not handled properly with current implementation.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

drivers/modem/simcom-sim7080.c

diff --git a/drivers/modem/simcom-sim7080.c b/drivers/modem/simcom-sim7080.c
index dfcfb49..958b008 100644
--- a/drivers/modem/simcom-sim7080.c
+++ b/drivers/modem/simcom-sim7080.c
@@ -447,6 +447,7 @@
  */
 static ssize_t offload_sendmsg(void *obj, const struct msghdr *msg, int flags)
 {
+	struct modem_socket *sock = obj;
 	ssize_t sent = 0;
 	const char *buf;
 	size_t len;
@@ -458,6 +459,17 @@
 		return -EAGAIN;
 	}
 
+	if (sock->type == SOCK_DGRAM) {
+		/*
+		 * Current implementation only handles single contiguous fragment at a time, so
+		 * prevent sending multiple datagrams.
+		 */
+		if (msghdr_non_empty_iov_count(msg) > 1) {
+			errno = EMSGSIZE;
+			return -1;
+		}
+	}
+
 	for (int i = 0; i < msg->msg_iovlen; i++) {
 		buf = msg->msg_iov[i].iov_base;
 		len = msg->msg_iov[i].iov_len;