tc_util: detect overflow in get_size

This detects overflow during parsing of value using get_size:

eg. running:

$ tc qdisc add dev lo root cake memlimit 11gb

currently gives a memlimit of "3072Mb", while with this patch it errors
with 'illegal value for "memlimit": "11gb"', since memlinit is an
unsigned integer.

Signed-off-by: Odin Ugedal <odin@ugedal.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

tc/tc_util.c

@@ -385,6 +385,11 @@ int get_size(unsigned int *size, const char *str)
 	}
 
 	*size = sz;
+
+	/* detect if an overflow happened */
+	if (*size != floor(sz))
+		return -1;
+
 	return 0;
 }