net_sched: htb: fix a typo in htb_change_class()

Fix a typo added in commit 56b765b7 ("htb: improved accuracy at high
rates")

cbuffer should not be a copy of buffer.

Signed-off-by: Vimalkumar <j.vimal@gmail.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Cc: Jiri Pirko <jpirko@redhat.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>