ieee802154: add proper length checks to header creations

Have mac802154 header_ops.create fail with -EMSGSIZE if the length
passed will be too large to fit a frame. Since 6lowpan will ensure that
no packet payload will be too large, pass a length of 0 there. 802.15.4
dgram sockets will also return -EMSGSIZE on payloads larger than the
device MTU instead of -EINVAL.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ieee802154/dgram.c b/net/ieee802154/dgram.c
index 55f2dc4..4c47154 100644
--- a/net/ieee802154/dgram.c
+++ b/net/ieee802154/dgram.c
@@ -233,7 +233,7 @@
 
 	if (size > mtu) {
 		pr_debug("size = %Zu, mtu = %u\n", size, mtu);
-		err = -EINVAL;
+		err = -EMSGSIZE;
 		goto out_dev;
 	}