clock: turn expensive division into multiply + cheap division

clock: turn expensive division into multiply + cheap division

On x86-64, dividing by a variable turns into a hugely expensive
divq. It's much cheaper to invert the division. Instead of
dividing clocks by clocks-per-usec, multiply by a 16M/clocks-per-usec
constant instead.

Signed-off-by: Jens Axboe <axboe@kernel.dk>