um: Add checks to mtu parameter parsing

Adds a sanity check to the parsing of mtu command line param
Signed-off-by: NAnton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: NRichard Weinberger <richard@nod.at>

arch/um/drivers/vector_kern.c

@@ -121,7 +121,8 @@ static int get_mtu(struct arglist *def)
 
 	if (mtu != NULL) {
 		if (kstrtoul(mtu, 10, &result) == 0)
-			return result;
+			if ((result < (1 << 16) - 1) && (result >= 576))
+				return result;
 	}
 	return ETH_MAX_PACKET;
 }