net: Split msg_iovlen check in recvmsg into each protocol

To prepare for supporting multiple iov in each protocol.

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

net/usrsock/usrsock_recvmsg.c

@@ -222,6 +222,11 @@ ssize_t usrsock_recvmsg(FAR struct socket *psock, FAR struct msghdr *msg,
   socklen_t outaddrlen = 0;
   ssize_t ret;
 
+  if (msg->msg_iovlen != 1)
+    {
+      return -ENOTSUP;
+    }
+
   if (fromlen)
     {
       if (*fromlen > 0 && from == NULL)