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>