net/sched: fix type of htb statistics

tokens and ctokens are defined as s64 in htb_class structure,
and clamped to 32bits value during netlink dumps:

cl->xstats.tokens = clamp_t(s64, PSCHED_NS2TICKS(cl->tokens),
                            INT_MIN, INT_MAX);

Defining it as u32 is working since userspace (tc) is printing it as
signed int, but a correct definition from the beginning is probably
better.

In the same time, 'giants' structure member is unused since years, so
update the comment to mark it unused.

Signed-off-by: Florent Fourcot <florent.fourcot@wifirst.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>