[NET]: Avoid divides in net/core/gen_estimator.c

We can void divides (as seen with CONFIG_CC_OPTIMIZE_FOR_SIZE=y on x86)
changing ((HZ<<idx)/4) to ((HZ/4) << idx)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>