[IPROUTE]: Introduce tc_calc_xmitsize and use where appropriate

Add tc_calc_xmitsize() as complement to tc_calc_xmittime(), which calculates
the size that can be transmitted at a given rate during a given time.

Replace all expressions of the form "size = rate*tc_core_tick2usec(time))/1000000"
by tc_calc_xmitsize() calls.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>